Democrats' Senate majority faces a midterm shrinkage Tuesday, a further complication for President Barack Obama's agenda, even if Republicans fall short of seizing control of the 100-member chamber.
Republicans must pick up 10 seats to regain the majority they lost four years ago. Analysts in both parties consider that a tough task. The GOP would have to win every toss-up race, plus score upsets in California, Washington or perhaps Connecticut; Obama coasted in those states in 2008.
Republicans seem almost certain to pick up Senate seats in North Dakota and Indiana -- where veteran Democrats are retiring -- and in Arkansas, where two-term Sen. Blanche Lincoln consistently has trailed Republican John Boozman in polls. Obama lost Arkansas by 20 percentage points in 2008.
These races could have long-term implications because Democrats may have trouble retaking those Senate seats six years and 12 years from now.
Democrats privately acknowledge they have slim chances of winning any GOP-held seats this year, despite earlier hopes in Ohio, New Hampshire, Missouri and Kentucky. Republicans are reveling in their good fortunes.
"At the start of this election cycle, most political experts were predicting additional gains in the Senate by the Democrats," said GOP spokesman Brian Walsh.
The opposite is true now.
Leaders in both parties say four fiercely contested races could go either way: Nevada, Colorado, Illinois and Pennsylvania.
Nevada has gotten the most national attention. Senate Majority Leader Harry Reid's popularity has sagged lately, and he seemed almost doomed a year ago. But Republicans nominated tea party favorite Sharron Angle, a relative newcomer whose inexperience and libertarian views have raised eyebrows.
An Angle triumph would mark the second time in six years that the Senate's Democratic leader lost a re-election bid amid charges that he became too focused on Washington. Tom Daschle of South Dakota fell victim in 2004.
Another tea party favorite, Ken Buck, is running a strong race in Colorado against Democratic Sen. Michael Bennet. Bennet was appointed to the seat, and is running his first campaign, which has impressed political pros in both parties.
The race to fill the open Illinois Senate seat once held by Obama has an unusually large number of undecided voters in the final days, perhaps reflecting both nominees' flaws. Republican Rep. Mark Kirk, a five-term House member, has struggled to overcome false statements he made about his military record.
Democrat Alexi Giannoulias, the state treasurer, has had to answer questions about ethical and legal questions surrounding his family's failed Chicago bank. Obama made a final weekend appearance with Giannoulias in Chicago.
In Pennsylvania, Democratic Rep. Joe Sestak defied his party's establishment by beating Republican-turned-Democratic Sen. Arlen Specter in the Senate primary. The GOP nominee is Pat Toomey, who advocates steep tax cuts and less regulation for businesses. Of the four on-the-bubble Senate races, Republicans feel most optimistic about Pennsylvania.
The biggest wild card entering the campaign's final hours is Alaska, where a tumultuous three-way race conceivably could fall to Democrat Scott McAdams. Republican Sen. Lisa Murkowski is trying to keep her seat with a write-in campaign after losing the GOP primary to tea party newcomer Joe Miller, who has stumbled in recent weeks.
Otherwise, the chief focus is on eight Democratic-held Senate seats that have been sharply contested for months.
Polls suggest that after Lincoln, the most imperiled Democratic senator is Russ Feingold of Wisconsin. The three-term senator, best known for revising campaign finance laws, was not seen as particularly vulnerable a few months ago.
But Republican challenger Ron Johnson, a political newcomer, turned Feingold's seniority against him in this season of antiestablishment fervor. His well-run campaign, coupled with Wisconsin's sluggish economy, have put Feingold in a hole.
If Democrats are deeply worried about Feingold, Republicans similarly feel that a once-promising chance may be slipping away in West Virginia, for the seat long held by the late Robert Byrd. The Democratic nominee, Gov. Joe Manchin, is popular. His state voted heavily against Obama, however, and the president's approval ratings there remain low.
Republican nominee John Raese has tried to tie Manchin tightly to Obama. But revelations that Raese's wife lives and votes in Florida, plus a GOP TV ad production that sought "hicky" actors to portray West Virginia voters, seem to have taken a toll.
Republican chances are dimmer against veteran senators in California and Washington.
Three-term Sen. Barbara Boxer of California faces Republican former business executive Carly Fiorina, who says Boxer is too liberal and too steeped in Washington for the times.
In Washington state, three-term Sen. Patty Murray is trying to hold off Republican Dino Rossi, a former state senator who made two unsuccessful bids for governor.
GOP officials say their nominees trail in those two states, but an upset is possible.
Regardless how these races turn out, the Senate will see several new members. Republicans expected to win include Boozman of Arkansas, Marco Rubio of Florida, Rand Paul of Kentucky, Jerry Moran of Kansas, John Hoeven of North Dakota, Roy Blunt of Missouri, Rob Portman of Ohio, Kelly Ayotte of New Hampshire, Mike Lee of Utah and Dan Coats of Indiana (who was a senator from 1988 to 1999).
New Democrats are likely to be Chris Coons of Delaware and Richard Blumenthal of Connecticut.
Wins by Buck, Angle, Fiorina or Rossi would expand the freshman class. Illinois, Pennsylvania and West Virginia will send rookies, no matter who wins there.
In Alaska, tea partier Miller seemed poised to join that group after stunning Murkowski in the GOP primary. But a series of gaffes and negative reports about his background sent Miller reeling. Murkowski hopes to become the second person to be elected to the Senate through write-in votes, after Strom Thurmond in 1954.
If Republicans come within a seat of taking the Senate majority, they may spend years second-guessing the Delaware primary. Popular GOP congressman Mike Castle was favored to win Vice President Joe Biden's former Senate seat. But tea party upstart Christine O'Donnell beat him in the Republican primary, only to see her fortunes quickly fall, despite assuring voters she is "not a witch."
Democratic spokesman Eric Schultz said Republicans shouldn't be too cocky.
"Like the weather," he said, "politics can be difficult to predict, and we believe Democrats are going to surprise people on Tuesday."