The presidential race isn't the only unpredictable war for control of Washington this year. Keep an eye on the U.S. Senate.
Expectations of a Republican takeover, which were widespread over the summer, are fading. Now the Democrats could retain their majority. Either way, it's close, and no one can safely say which party will have a Senate majority after the Nov. 6 elections.
Among the changes in the landscape: President Barack Obama has an edge over Republican Mitt Romney in national polls as well as in key swing states such as Virginia and Nevada, suggesting that Democrats might turn out in bigger numbers and also vote for Democratic Senate candidates.
Another: The once-vulnerable seat held by Sen. Claire McCaskill of Missouri now appears safely Democratic since the Republican nominee, U.S. Rep. Todd Akin, said this summer that women rarely got pregnant in cases of "legitimate rape."
Republicans need a net gain of four seats to take control of the Senate if Obama wins, three if Romney is elected – since his vice president would break a tie. Democrats now control 53 seats, but 23 of them are at stake. Republicans need to defend only 10.
Adding to the uncertainty: 2012 isn't shaping up as a "wave election," when voters routinely sweep candidates of one political party out of office.
Instead, "there's hand-to-hand combat, state by state," said Nathan Gonzales, a political analyst at the nonpartisan Rothenberg Political Report.
He and other experts advise watching 10 races. Five are genuine tossups, too close to call and likely to hinge on the right last-minute ads or debate quirks, or whether partisans do turn out in big numbers for Romney or Obama.
Rothenberg and The Cook Political Report, a nonpartisan firm that follows campaigns closely, agree that at the moment, Montana, North Dakota and Virginia, now held by Democrats, and Massachusetts and Nevada, now Republican seats, are too close to call.
A second group of contests have potential to become volatile: Hawaii, Wisconsin and Missouri, all now Democratic seats, and Indiana, now held by a Republican. Also in the mix is the Connecticut seat held by retiring Sen. Joe Lieberman, an independent who caucuses with Democrats.
The dominant issue in all these races is the economy and who can best fix it. One factor that doesn't appear to be driving Senate campaigns is the public's disdain for Congress. Gallup found earlier this month that Congress' approval rating had sunk to 13 percent, its lowest figure this late in an election year since such polls began in 1974.
With Democrats now in charge of the White House and Senate, and Republicans holding a majority in the House of Representatives, voters tend to see Washington inertia as the result of gridlock, not ineptitude. Voters figure that if only they can elect more people from the party they prefer, Washington will work.
"Everyone thinks their party can do better," said David Damore, an associate professor of political science at the University of Nevada Las Vegas.
Here are the races to watch closely:
Montana
Two candidates who've successfully run statewide, freshman incumbent Democratic Sen. Jon Tester, and Republican Denny Rehberg, the state's six-term at-large congressman, are in a slugfest that's impossible to call. The Cook Political Report's Jennifer Duffy termed this race "a marathon on a treadmill."
Old-fashioned hand-shaking and crowd-pleasing in a state where the biggest city, Billings, has a population of about 105,000 are likely to determine the outcome.
North Dakota
Once expected to move into Republican control, the seat held by retiring Senate Budget Committee Chairman Kent Conrad, a Democrat, has turned this small state into a political combat zone.
Democrat Heidi Heitkamp, a former attorney general who's seasoned in statewide races, is credited with running an energetic campaign against freshman Republican U.S. Rep. Rick Berg. Berg stays close as he hammers the unpopular Obama.
Virginia
Two former governors are competing in a state that's already getting lots of attention because it's a presidential-race tossup.
Democrat Tim Kaine has pulled slightly ahead in most polls, but Republican George Allen has won statewide twice before, for governor in 1993 and Senate in 2000. Virginia is hard to handicap, since demographics are changing rapidly as northern Virginia counties attract more professionals who've been trending Democratic. Turnout for Obama or Romney could make the difference here.
Nevada
UNLV's Damore described the race as a choice between unpopular alternatives. Incumbent Republican Sen. Dean Heller has been slipping; his 9-percentage-point lead in July's Rasmussen poll shrank to 1 last week.
Democratic U.S. Rep. Shelley Berkley has faced ethics questions over her efforts to affect federal involvement in kidney health policy. Her husband is a well-known Las Vegas nephrologist. Berkley has said her interest was assuring quality health care for her constituents.
Nevada's economy has been as dismal as any state's in the nation, but what might make the difference are immigration issues, particularly since 15 to 20 percent of the vote may come from the Latino population. Polls have differed on which candidate has more support among Hispanics.
Massachusetts
Democrat Elizabeth Warren led incumbent Republican Sen. Scott Brown by an average of 2.4 percentage points in polls taken last week, according to RealClearPolitics, a website that compiles such data.
Warren has been gaining slowly, and David Paleologos, the director of the Suffolk University Political Research Center, attributed her progress to Democrats warming to the fiery consumer advocate. Brown faces another problem: Though Romney is a former governor of the state, he isn't particularly popular. Last week's Suffolk poll found Obama ahead by 64-31 percent.
Massachusetts voters, though, are known for embracing moderate Republicans, and the poll found that half liked the idea of bipartisan representation in Washington.
Brown has been careful to keep a distance from Romney. "I'm Scott Brown. He's Mitt Romney," he said. "People know who I am. I'll let my record speak for itself."
In the other Senate races being watched, turnout for the presidential election could make a difference. In Connecticut, for instance, the Sept .11-16 University of Connecticut/Hartford Courant poll had Obama up by 21 percentage points. Yet Republican Senate candidate Linda McMahon trailed Democratic U.S. Rep. Christopher Murphy by only 4.
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