WASHINGTON (CNN) -- Key victories in Senate races Tuesday mean Democrats will keep control of the Senate, CNN projects.
While poll results were still trickling in for some significant races, the Democratic wins will make it impossible for Republicans to regain control of the chamber.
"No chance we get there," a top GOP source told CNN. "We have (a) real possibility of going backwards."
According to CNN projections, Democratic candidate Elizabeth Warren defeated incumbent Republican Sen. Scott Brown in Massachusetts and Democratic Rep. Joe Donnelly scored a major upset in the Indiana Senate race when he won against tea party candidate Richard Mourdock.
Both projected wins are pickups of key Republican seats and critical to Democratic ambitions to maintain their majority control of the Senate.
Democrats also scored notable victories in Virginia, Ohio and Connecticut.
Former Gov. Angus King's victory in Maine's Senate race was another blow to Republicans. King, an independent who quickly rose to frontrunner status, won the seat vacated by retired moderate Republican Sen. Olympia Snowe.
King hasn't said which party he would caucus with in the Senate, but he is widely assumed to align with Democrats.
Considered a shoo-in for re-election, Snowe shocked her colleagues earlier this year when she suddenly announced that she would leave the Senate, which she described as hopelessly partisan.
Two years ago, Republicans had every reason to believe they could take back the Senate this year, after major midterm election gains. But heading into Tuesday, Democrats appeared well-positioned to retain their slim majority, and with it, the ability to influence much of the Washington agenda during the next two years.
There were 33 Senate seats at play on Tuesday.
Brown defeats Mandel in contentious Ohio Senate race
Going into Tuesday's vote, Republicans were protecting only 10 seats, while Democrats were defending 23, many in narrowly divided swing states. In addition, several veteran Democratic incumbents, mostly moderates, announced they would retire, making it potentially even easier for Republicans to win those seats.
Republicans have encountered their own problems.
In August, the campaign of Rep. Todd Akin nearly collapsed after the Missouri Republican's comments about "legitimate rape" and his suggestion that women could biologically prevent pregnancy if they are raped. Until then, Republicans believed Akin would defeat Democratic Sen. Claire McCaskill, who polls showed was not very popular after just one term.
McCaskill defeated Akin at the polls Tuesday, CNN projects.
Mourdock sparked a similar controversy last month when he defended his opposition to abortion even in the case of rape because, "I think even when life begins in that horrible situation of rape, that it is something God intended to happen."
Operatives from both parties agreed that many Senate races would be determined by which presidential candidate carries the state, particularly in key battleground states like Florida, Virginia, Pennsylvania, Wisconsin, and Nevada.
Key Senate race snapshots
Arizona: Rep. Jeff Flake (R) vs. Richard Carmona (D)
Open seat -- Sen. Jon Kyl (R) is retiring
This race turned more competitive than originally expected. Democratic nominee Dr. Richard Carmona, a Vietnam veteran and a former U.S. surgeon general under President George W. Bush, proved to be a formidable opponent to six-term Rep. Jeff Flake, whose tough August primary for the GOP nomination left him bruised as he began the general election.
Tightening polls caused both campaigns to go negative, with Flake accusing Carmona of having anger issues over an incident in which a former HHS official accused him of banging on her door in the middle of the night and scaring her family (Carmona denied the incident ever occurred). Carmona accused Flake of not supporting veterans as a congressman (Flake said Carmona is cherry-picking votes and not looking at his entire record).
Democrats thought their candidate's strengths and the state's growing Hispanic population will lead to their party's first successful Senate election since 1988. But Republicans pointed to no significant changes in Hispanic voting records, Flake's endorsements from Sens. Kyl and John McCain, and the state's traditional GOP support as reasons for a Flake victory.
Connecticut: Rep. Chris Murphy (D) vs. Linda McMahon (R)
CNN projection: Rep. Chris Murphy wins
In the closely watched senate race in Connecticut, Democratic Rep. Chris Murphy defeated Republican nominee and professional wrestling executive Linda McMahon, according to a CNN projection.
Murphy will take over the Senate seat being vacated by retiring Sen. Joe Lieberman, an Independent who sided with Democrats.
His win preserves a seat for Democrats who feared losing it to the wealthy McMahon, who spent over $90 million in the last two elections to try to win a Senate seat.
The race was marked by heavy negative advertising.
Indiana: Richard Mourdock (R) vs. Rep. Joe Donnelly (D)
CNN projection: Rep. Joe Donnelly wins
Democrats scored a major upset in the Indiana Senate race, scooping up a seat in the deeply red state Republicans had expected to hold.
The campaign appeared to turn for the Democrats after the GOP candidate made controversial comments about rape during a televised debate.
Mourdock, the State Treasurer, was backed by both tea party supporters and the Republican establishment when he defeated in the primary longtime GOP moderate Sen. Richard Lugar. When that happened, Democrats thought Donnelly, who is conservative and pro-life, had an outside chance of winning the seat.
Mourdock gave Donnelly an opening when during a debate he defended his opposition to abortion even in the case of rape because, "I think even when life begins in that horrible situation of rape, that is something God intended to happen." Top Democrats and Republicans criticized the remark and within days, Donnelly took a substantial lead in one respected Indiana poll.
Maine: Charlie Summers (R) vs. Cynthia Dill (D) vs. former Gov. Angus King (I)
CNN projection: Former Gov. Angus King wins
Former Gov. Angus King, an Independent, steadfastly has refused to say if he will organize with Democrats or Republicans when he gets to Washington. If the Senate is narrowly divided after Tuesday's election, he could determine which party gets majority control.
King will replace Sen. Olympia Snowe, herself an independent-minded Republican who stunned her colleagues this year when she announced she would retire from the Senate, which she complained has become increasingly partisan.
Democrats are convinced King will side with them. As evidence, they point to King's support for Obama and many of his policies. In a sign of that confidence, national Democrats ignored the Democratic candidate Dill and spent heavily against Summers in order to help King.
Republicans also believe that King will caucus with Democrats, so they spent millions to defeat him.
But an aide told CNN last week that King still has not made up his mind and won't until he gets to Washington to see the state of play.
Massachusetts: Sen. Scott Brown (R) vs. Elizabeth Warren (D)
CNN projection: Elizabeth Warren wins
In a key pick-up of a Republican Senate seat, Warren defeated Brown Tuesday, according to a CNN projection.
Warren, a Harvard University law professor and former Obama administration financial consumer advocate, beat the freshman senator who was seeking a full term to the seat he won two years after the death of Democratic senator Ted Kennedy. Brown, the first GOP Senator from deep blue Massachusetts in decades, billed himself as an independent who voted for the needs of Massachusetts, not the demands of either party.
The Warren-Brown race was one of the costliest and nastiest campaigns this cycle. It was marked by Brown's repeated accusations that Warren claimed to be a minority in order to advance her career. She denied that was her motive for listing her ancestry as Native American, saying she was told about her heritage by her family.
Montana: Sen. Jon Tester (D) vs. Rep. Dennis Rehberg (R)
Democrat Jon Tester knew he'd have an uphill battle to a second term against six-term Rep. Denny Rehberg. Both are known quantities with high favorables in the state, though neither was able to crack 50% in the polls. Both campaigns went negative in the fight over Social Security, and both said the other is distorting their views.
Like the Massachusetts race, there were few undecideds left, except here Republicans were hoping that worked in their favor.
Nebraska: Former Sen. Bob Kerrey (D) vs. Deb Fischer (R)
Open seat -- Sen. Ben Nelson (D) is retiring
Republicans began eyeing two-term Democrat Ben Nelson's seat even before he announced his retirement. With Nelson out, Democrats pinned their hopes on Bob Kerrey, the former governor and two-term senator. Although a well-known figure in Nebraska, Kerrey spent most of his post-Senate career outside of the state, primarily in New York City where he served as president of The New School.
The conservative super PAC American Crossroads began running ads against Kerrey before he even declared his candidacy. The Republican nominee is state Rep. Deb Fischer, who scored a surprising win in a crowded GOP primary. Kerrey has an uphill battle to keep the seat blue; Fischer has been leading with at least 50% in both independent and partisan polls.
Nevada: Sen. Dean Heller (R) vs. Rep. Shelley Berkley (D)
Democrats' strength in Nevada was being put to the test once again in a Senate election. With voter registration numbers on their side, seven-term Democratic Rep. Shelley Berkley hoped to deny Republican Sen. Dean Heller a full Senate term (Heller was appointed to the seat after Republican John Ensign resigned).
A House ethics investigation and a lackluster debate performance kept Berkley below Heller in the most recent polls. Heller's strong fundraising ability also helped him keep the advantage in a state with a heavy Latino population that overwhelmingly votes Democratic. Turnout for the presidential race could have the largest impact on the outcome of this race.
North Dakota: Heidi Heitkamp (D) vs. Rep. Rick Berg (R)
Open seat -- Sen. Kent Conrad (D) is retiring
Former state Attorney General Heidi Heitkamp gave Democrats their best opportunity to keep the seat of retiring five-term Sen. Kent Conrad. Republicans hoped first-term Rep. Rick Berg will repeat the party's success of 2010, when they took over retiring Democrat Byron Dorgan's seat.
The state has a history of split-ticket voting, giving Heitkamp an opportunity to show her independence from national Democrats. She's publicly disagreed with Barack Obama on issues like energy, which she points to as evidence she won't be a rubber stamp. She enjoyed high favorables and solid support from Republican ticket-splitters and has hammered Berg for his connection to a controversial real estate company.
With the lowest unemployment rate in the country, North Dakotans had their choice between two candidates with strong statewide appeal.
Ohio: Sen. Sherrod Brown (D) vs. Josh Mandel (R)
CNN projection: Sen. Sherrod Brown wins
Primarily because of those frequent trips from President Barack Obama and Republican candidate Mitt Romney, the contest between Brown and Mandel played out in Ohio against the backdrop of the presidential election.
Fueled by Super PAC money and advertising, the race got nasty at times. In total, $67 million dollars was spent in the election, according to the Center for Responsive Politics. Much of that - $34 million -- was sent by outside groups advocacy groups.
At the three debate, Mandel and Brown battled over the auto bailout, abortion and each other's record. The testiness of the race spilled out on the debate stage -- at one debate Mandel told Brown to "calm down" and called him "a liar."
Brown was first elected to the Senate in 2006, during a wave of Democratic victories in the Senate and House. Throughout most of the 2012 race, especially in September and October, Brown led Mandel by around 10 percentage points in independent poling.
Pennsylvania: Sen. Bob Casey (D) vs. Tom Smith (R)
CNN projection: Sen. Bob Casey wins
Though recent polls have given Republicans hope where they previously had little, freshman Democratic Sen. Bob Casey of Pennsylvania was re-elected Tuesday.
GOP candidate Tom Smith, a wealthy business executive, lagged far behind Casey for much of the race.
In the last few weeks, he spent heavily on advertising and was able to narrow the polls. But he never took the lead
Virginia: Former Gov. Tim Kaine (D) vs. former Gov./Sen. George Allen (R)
CNN projection: Gov. Tim Kaine wins
Allen conceded in a Tuesday night speech and pledged his support to Kaine.
"I congratulated him and pledged my support and cooperation as he undertakes the solemn vows of representing the people of Virginia during very difficult times in our nation's history," Allen said. "We haven't succeeded my friends in winning this election."
The race pitted two former Virginia governors against one another and each pulled from their experience in the state house throughout the race.
Though, Kaine linked himself to Obama and the president's campaigning in Virginia, he also painted himself as someone who had worked with both Democrats and Republicans. Allen, however, tried to turn that link against Kaine - tying him to Obama and the Democratic Party, where the former democratic governor served as chairman.
Making that case was costly. When outside and candidate spending was added together, the Kaine-Allen race was the most expensive race in the country. Outside groups, on their own totaled over $50 million, according to the Center for Responsive Politics.
Wisconsin: Rep. Tammy Baldwin (D) vs. former Gov. Tommy Thompson (R)
Open seat -- Sen. Herb Kohl (D) is retiring
Former Gov. Tommy Thompson won a tough Republican primary to take on seven-term Democratic Rep. Tammy Baldwin for retiring incumbent Herb Kohl's Senate seat. Thompson's attacks on Baldwin as being too liberal haven't had the impact he was hoping. The polls were very close, but Baldwin pulled slightly ahead recently, though not outside the margin of error.
Unless vice presidential candidate and native son Paul Ryan brings out strong turnout for the party, the state's historic Democratic tilt in presidential elections could help Baldwin succeed to becoming the first openly gay senator.