WASHINGTON — 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 now control 53 of the Senate’s 100 seats. Twenty-three of those Democratic seats are up for re-election, compared with just 10 in Republican hands. Republicans need a net gain of four seats for their first majority in six years, 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.
Election night’s trend should be apparent early. If Democrats hold the seat in Connecticut and win Massachusetts, they have a cushion that will help offset any Republican gains in the Midwest. If Democrats lose one or both of the New England seats, the suspense shifts westward.
Here’s a look at races to watch election night:
DEMOCRATIC SEATS THAT COULD GO REPUBLICAN
CONNECTICUT: Incumbent Sen. Joe Lieberman, independent, retiring. President Barack Obama’s coattails could matter. The state has a long history of choosing moderate Republicans and independent thinkers for statewide office, and Republican candidate Linda McMahon is trying to cast herself as heir to that legacy.
Democratic Rep. Chris Murphy has been dogged by McMahon’s criticism of being frequently absent for committee meetings. But he’s known for his organizing skills and benefits from Obama’s overwhelming popularity in the state.
VIRGINIA: Incumbent Sen. Jim Webb, Democrat, retiring. Polls show former Gov. Tim Kaine, a Democrat, gaining on Republican nominee George Allen.
Virginia’s a swing state, though, which means the Romney and Obama organizations have massive turnout operations. Allen has won statewide twice before, for governor in 1993 and the Senate seven years later.
WISCONSIN: Incumbent Sen. Herb Kohl, Democrat, retiring. Once regarded as an easy Democratic win, former Republican Gov. Tommy Thompson and Democratic Rep. Tammy Baldwin are locked in an ugly duel, where negative ads are blanketing local airwaves.
This will be a turnout war, said Charles Franklin, director of the Marquette University Law School poll. While Democrats have a well-honed ground game, Republicans can tap into Gov. Scott Walker’s organization, which five months ago helped him turn back a recall effort.
MISSOURI: Incumbent Sen. Claire McCaskill, Democrat. McCaskill got the break of a political lifetime 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 drop out.
He didn’t, and some polls show him within striking distance of McCaskill. They’re waging a particularly fierce fight for the votes of women.
NORTH DAKOTA: Incumbent Sen. Kent Conrad, Democrat, retiring. Republicans eyed this race as one of their best bets to gain a seat.
Republican Rick Berg started with a huge advantage in this heavily Republican state, but he has found former Attorney General Heidi Heitkamp and her high-energy campaign formidable. Berg has stayed ahead in polling but has had trouble getting above 50 percent. The difference could be Romney’s coattails.
MONTANA: Incumbent Sen. Jon Tester, Democrat. Two immensely popular statewide winners — Tester and Rep. Denny Rehberg, the Republican who’s won the state’s at-large seat six times — are engaged in the political equivalent of hand-to-hand combat.
Tester is one of the Senate’s more moderate Democrats, while Rehberg has a more conservative record.
REPUBLICAN SEATS THAT COULD GO DEMOCRATIC
MASSACHUSETTS: Incumbent Sen. Scott Brown, Republican. Brown has tried hard to put distance between himself and his party’s conservative wing, as well as from Romney, governor from 2003 to 2007.
But Romney is not popular, and polls show Obama with a 2-to-1 lead. Democrat Elizabeth Warren, known for her aggressive consumer advocacy, now appears to be the favorite.
INDIANA: Incumbent Sen. Richard Lugar, Republican, defeated in primary. This was supposed to be a safe Republican seat, but when the popular Lugar lost to insurgent Richard Mourdock, Democrats saw an opening.
Mourdock, a favorite of the GOP conservative wing, has had trouble selling himself as a mainstream candidate. He hurt himself last month when he said that “even when life begins in that horrible situation of rape, that it is something that God intended to happen.” He later apologized for offending anyone, but the political damage lingers.
ARIZONA: Incumbent Sen. Jon Kyl, Republican, retiring. A seat that once looked safe for Rep. Jeff Flake, the GOP nominee and a longtime favorite of conservatives, is now a tossup.
The state hasn’t elected a Democrat to the Senate since 1988. But Richard Carmona, former surgeon general in President George W. Bush’s administration, has waged a vigorous campaign, appealing particularly to moderates and the Latino community. Turnout will matter here.
NEVADA: Incumbent Sen. Dean Heller, Republican. Heller has inched ahead of Rep. Shelley Berkley in polls. The Democrat has faced ethics questions over her efforts to affect federal involvement in kidney health policy. Her husband, a physician, is a kidney specialist. She said her interest was assuring quality health care for her constituents.
The outcome rests on several variables. This is a swing presidential state, meaning both sides will be making strong turnout efforts. The Latino population is heavily Democratic, but turnout is uncertain. Senate Majority Leader Harry Reid’s organization is known for its skill at mobilizing voters, but Nevada’s horrid economy could turn voters away from Democrats.