(NBC) NBC News projected that Democrats held 51 seats in the next Senate, plus the consistent support of Independent Bernie Sanders of Vermont, who won re-election.
Three races were still too close to call, meaning the Democrats could strengthen their grip on the upper chamber.
With the House remaining in Republican hands, the makeup of the government will remain static: President Barack Obama was re-elected, but he will have to contend with a divided Congress for four more years.
"Things like this are what happens when your No. 1 goal is to defeat the president and not work to get legislation passed," Senate Democratic leader Harry Reid of Nevada said in a statement.
In a statement of his own, Republican leader Mitch McConnell of Kentucky challenged Obama to "propose solutions that actually have a chance of passing the Republican-controlled House of Representatives and a closely divided Senate."
"To the extent he wants to move to the political center, which is where the work gets done in a divided government, we'll be there to meet him halfway," McConnell said.
View complete Senate election results
The Democrats clung to control on the back of four victories in states that had been statistical ties in pre-election polls:
Harvard University law professor Elizabeth Warren ousted Republican Sen. Scott Brown in Massachusetts, NBC News projected, negating Republican state legislator Deb Fischer's victory over former Sen. Bob Kerrey for the open seat of retiring Democratic Sen. Ben Nelson. Massachusetts results | Nebraska results
Democratic former Virginia Gov. Tim Kaine beat former Republican Gov. George Allen, NBC News projected, keeping the seat held by the retiring Sen. Jim Webb in Democratic hands. See results
Democratic Rep. Joe Donnelly of Indiana defeated Republican state Treasurer Richard Mourdock to claim the open seat held by Republican Dick Lugar, NBC News projected. Mourdock had been favored until he drew national opposition for having said in a debate last month that he believed that pregnancies resulting from rape were a "gift from God" and shouldn't be terminated. See results
Democratic Sen. Claire McCaskill held on to her seat in Missouri after Republican Rep. Todd Akin made similar comments in a TV interview in August, suggesting that women's bodies could "shut down" a pregnancy that was the result of a "legitimate rape." See results
As expected, Rep. Martin Heinrich, D-N.M., defeated Republican former Rep. Heather Wilson to win the open seat of retiring Republican Jeff Bingaman, and Rep. Tammy Baldwin, D-Wis., beat Republican former Gov. Tommy Thompson to become the nation's first openly gay senator, NBC News projected. Thompson, who served as secretary of Health and Human Services in the administration of former President Geoorge W. Bush, announced his retirement from politics in his concession speech.
And Democrats could claim a bigger majority.
Former Maine Gov. Angus King, running as an independent, won an open Senate seat that had been held by Republican Olympia Snowe, NBC News projected. King could vote with the Democrats; he hasn't said which party, if any, he will side with.
The three still-undecided races, meanwhile, also could go either way:
Sen. Jon Tester, D-Mont., held a 14-point lead over Republican Rep. Denny Rehberg in a race that NBC News said was also too early to call. See results
Sen. Dean Heller, R-Nev., who was appointed to the seat when former Republican Sen. John Ensign resigned, held a slim lead in pre-election polls to keep his seat over Democratic Rep. Shelley Berkley. NBC News said that race was too early to call. See results
And Rep. Rick Berg, R-N.D., was closely trailing Democratic former Attorney General Heidi Heitkamp in a race NBC News was describing as too close to call.
Democrats control 53 seats in the current Senate (that number includes Sanders and Joe Lieberman of Connecticut, who also generally votes with them); Republicans hold 47. Ten senators weren't seeking re-election this cycle — the most since 1996. In addition, Lugar lost to Mourdock in the Indiana Republican primary, meaning at least 11 new faces will join the Senate on Jan. 2.
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Warren's victory was particularly sweet for Democrats, for whom she was a hero as the architect of Obama's U.S. Consumer Financial Protection Bureau.
Warren beats Brown in costliest-ever Senate race
"You took on the powerful Wall Street banks and special interests, and you let them know you want a senator who will be out there fighting for the middle class all of the time," she told cheering supporters in Boston.
The races in Missouri and Indiana were also closely watched because of the controversies generated by Akin's and Mourdock's comments on abortion.
McCaskill reveled in her victory, giving supporters a beaming I-told-you-so speech in St. Louis.
"They all said it's over — it's done, it's too red, it's just too red," she said. "There is no way that Claire McCaskill can survive. Well, you know what happened? You proved them wrong."
Akin told supporters in Missouri that he had called to congratulate McCaskill, but he sounded a defiant note:
Todd Akin says that called Claire McCaskill to concede after being defeated in the Missouri Senate race.
"I also think, in the circumstances that we've all been through, that it is particularly appropriate to thank God, who makes no mistakes and is wiser than we are," Akin said.
"... Washington, D.C.'s first questions shouldn't be what's politically expedient, but what's right," he said. "Washington doesn't need more money. It needs more courage."
Donnelly, meanwhile, stressed bipartisanship, telling supporters in Indianapolis that he hoped to follow in the moderate shoes of two predecessors, Lugar and Democrat Evan Bayh.
"I say to all of my fellow Hoosiers out there: This isn't about politics. This isn't about one party or the other," Donnelly said.