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 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. They also warn it’s an unusually difficult year to figure, because the 2012 vote is not shaping up to be the kind of national referendum on an issue such as the Iraq war, or the economy, that gives one political party an advantage.

“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.

“There’s a very high likelihood Democrats will hold onto the Senate, and it’s the Republicans’ own fault,” said Geoffrey Skelley, a political analyst at the Center for Politics at the University of Virginia.

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 west.

Here’s a look at races to watch election night, from coast to coast:

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 the 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, making the Senate race hard to call. 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 an effort to recall him.