Updated at 10:21 p.m. ET: Democrats won three races that Republicans had targeted as critical to their hopes of winning control of the Senate on Tuesday, making it appear increasingly likely that Congress would remain divided in January.
NBC News projected that Democrats won close contests in Missouri and Indiana, where the Republican candidates stumbled by making comments on abortion that many in both parties saw as extreme. And they gained another seat in Massachusetts, where Harvard University law professor Elizabeth Warren ousted Republican Sen. Scott Brown, NBC News projected.
Democratic Senate candidate Elizabeth Warren waves to supporters before voting in Cambridge, Mass., on Tuesday, Nov. 6. Photo: Sahnnon Stapleton/Reuters/NBCNews.com.
Former Maine Gov. Angus King, running as an independent, also won an open Senate seat that had been held by Republican Olympia Snowe, NBC News projected.
Maine results  | Massachusetts results  | Indiana results  | Missouri results 
Nebraska bucked the Democratic Senate tide, as Republican state legislator Deb Fischer defeated former Sen. Bob Kerrey to win the open seat of retiring Democratic Sen. Ben Nelson.
See results 
And Republican Ted Cruz defended a GOP seat in Texas by defeating Democrat Paul Sadler, NBC News projected. He will succeed retiring Republican Kay Bailey Hutchison. See results 
The results gave the Democrats a net gain of two seats as of 10:41 p.m. ET, severely hampering the Republican drive to wrest four seats from Democratic control to get to a 50-50 tie. That would give Republicans control if Mitt Romney wins the White House, because Republican Vice President Paul Ryan would be the tiebreaking vote.
In Indiana, Democratic Rep. Joe Donnelly defeated Republican state Treasurer Richard Mourdock in one of the two races that drew national attention from advocates for and against legal abortion. In a debate last month, Mourdock said he believed that pregnancies resulting from rape were a "gift from God" and shouldn't be terminated.
Similarly, Democratic Sen. Claire McCaskill held on to her seat in Missouri after Republican Rep. Todd Akin made comments in a TV interview suggesting that women’s bodies could "shut down" a pregnancy that was the result of a "legitimate rape."
View complete Senate election results 
Democrats control 53 seats in the current Senate (that number includes two independents, Vermont's Bernie Sanders and Joe Lieberman of Connecticut, who caucus and generally vote with them); Republicans hold 47. Ten senators weren't seeking re-election this cycle — the most since 1996. In addition, Dick Lugar, R-Ind., was defeated by Mourdock in his state's primary, meaning at least 11 new faces will join the Senate on Jan. 2.
The Republicans are projected to keep control of the House , and as recently as August, they looked like a legitimate bet to take over the Senate, too, running virtually dead even in races for five Democratic-held seats. But then Akin made his comments in a TV interview about abortion, and the landscape began to change.
The last round of tracking polls through the weekend indicated that McCaskill went into Election Day leading by an average of 6 percentage points.
Two other races, still to be decided Tuesday, also were razor-close:
Incumbent Republican Dean Heller, who was appointed to the seat when former Republican Sen. John Ensign resigned, held a 4-point average lead in the race to keep his seat over Democratic Rep. Shelley Berkley. See results 
Incumbent Jon Tester was the most endangered Democrat. The last round of pre-election polls showed him in a dead heat with Republican Rep. Denny Rehberg. See results