WASHINGTON — Democrats appear poised to retain control of the Senate, but this year's forecasts are full of more uncertainty than usual.
A host of unknowns could affect the 10 or so races too close to call: Turnout. Ground game. Last-minute ads. Presidential coattails. Weather.
Democrats hold 53 of the Senate's 100 seats. Twenty-three of the Democratic seats are up for re-election, compared with 10 in GOP hands. Republicans need a net gain of four seats for a majority, three if Mitt Romney wins, allowing a Vice President Paul Ryan to cast tie-breaking votes.
Independent experts agree Republicans could wind up with anywhere from a net gain of three seats to zero.
"Neither party has the wind at its back," said Jennifer Duffy, Senate analyst for the nonpartisan Cook Political Report. "It's all about candidates and their campaigns."
Part of the Republicans' challenge is that two seats the party counted on — Indiana and Missouri — are now tossups, at best.
Here's a look at some races to watch election night.
Democratic seats that could go Republican:
• Connecticut. Incumbent Sen. Joe Lieberman, an independent, is retiring. He caucused with Democrats in the Senate. Republican candidate Linda McMahon is trying to cast herself as a moderate. Democratic Rep. Chris Murphy is known for his organizing skills.
• Virginia. Democratic Sen. Jim Webb is retiring. Polls show former Gov. Tim Kaine, a Democrat, gaining on Republican nominee George Allen.
• Wisconsin. Democratic Herb Kohl is retiring. Once regarded as an easy Democratic win, former Republican Gov. Tommy Thompson and Democratic Rep. Tammy Baldwin are locked in a duel.
• Missouri. Democratic Sen. Claire McCaskill got a break when Rep. Todd Akin upset the Republican establishment's choices to win the nomination. He then said women rarely get pregnant in cases of "legitimate rape," sparking calls from top Republicans for him to leave the race. He didn't, and some polls show him within striking distance of McCaskill.
•North Dakota. Democratic Sen. Kent Conrad is retiring. Republican Rick Berg started with an advantage in the heavily Republican state, but he has found former Attorney General Heidi Heitkamp and her high-energy campaign formidable.
• Montana. Democratic Sen. Jon Tester and Republican Rep. Denny Rehberg are engaged in the political equivalent of hand-to-hand combat. Tester is one of the Senate's more moderate Democrats; Rehberg has a more conservative voting record.
Republican seats that could go Democratic:
• Indiana. Republican Sen. Richard Lugar was defeated in the primary. This was supposed to be a safe GOP seat, but Republican Richard Mourdock has had trouble selling himself as a mainstream candidate and Democratic Rep. Joe Donnelly held a lead in a poll released on Friday. Mourdock's comment during a televised debate that pregnancy resulting from rape is something "God intended" has come to define the race.
• Massachusetts. Democrat Elizabeth Warren, known for her aggressive consumer advocacy, appears to be the favorite over Republican Sen. Scott Brown, according to recent polls.
• Arizona. Republican Sen. Jon Kyl is retiring. A seat that once looked safe for Republican Rep. Jeff Flake is a tossup. Democrat Richard Carmona, surgeon general in President George W. Bush's administration, has waged a vigorous campaign.
• Nevada. Republican Sen. Dean Heller has inched ahead of Democratic Rep. Shelley Berkley in polls, but the outcome appears to rest on variables such as turnout and the Latino vote.