Whichever party controls the Senate after the November 4 ballotingRepublicans need a six-seat gain to win the majoritywill say much about what President Barack Obama can accomplish in the final two years of his presidency and the tone of the race to succeed him.
"The end of the 2014 general election does, in a sense, commence a beginning of the presidential primary phase," says New Hampshire Republican operative Rich Killion. "But an informal, unofficial opening to the process already is underway."
Here's a look at potential 2016 candidates and what to expect this fall:
HILLARY RODHAM CLINTON:
The former secretary of state's every word will be parsed for her future plans. But Clinton has been offering plenty of hints that she's preparing for another campaign.
Her biggest splash could come in Iowa, where she'll join her husband at Sen. Tom Harkin's annual steak fry fundraiser in Indianola on Sept. 14. The event is billed as a tribute to Harkin, but will generate wide interest as Clinton's first visit to Iowa since losing the 2008 caucuses.
Clinton has limited her campaign activity since leaving the State Department, but this fall should give voters a more concrete look at how she might present her candidacy. Her allies are wary of a "third Obama term" label, so Clinton's speeches and appearances offer a chance to distinguish herself from the president.
She will raise money for Democrats' four major campaign committees and could help several Senate campaigns where Obama remains a liability.
Vice President Joe Biden has not ruled out a third presidential bid and expects to be an active surrogate for Democrats this fall. Whether he'd challenge Clinton for the Democratic presidential nomination remains the big question.
Biden headlined high-profile meetings with young voters, liberals and African-Americans. He's also raised money for congressional candidates in Nevada and incumbent governors in Connecticut and Illinois. Biden is expected to visit New Hampshire, where he maintains ties to party activists, and Iowa, where Rep. Bruce Braley faces Joni Ernst in one of the top Senate battlegrounds.
Several Democrats are building for a national campaign in case Clinton doesn't run or considering a longshot challenge.
Maryland Gov. Martin O'Malley has been the most active, raising money for candidates in Iowa and New Hampshire and traveling to states with active mid-term contests.
Former Virginia Sen. Jim Webb recently traveled to Iowa. Vermont Sen. Bernie Sanders, an independent who caucuses with Democrats, plans to visit the Hawkeye State in mid-September. Massachusetts Sen. Elizabeth Warren has denied interest in the White House but would face pressure to run if Clinton doesn't.
More than seven years out of office, former Florida Gov. Jeb Bush has been quieter than some of his GOP colleagues as he focuses on his private business dealings.
He recently said he'd begin a more aggressive schedule to help Republicans this fall. He's set to headline a Florida fundraiser in late September to benefit top Republican Senate candidates, a group expected to include Cory Gardner in Colorado, Ernst in Iowa, Monica Wehby in Oregon and Tom Cotton in Arkansas.
Kentucky Sen. Rand Paul has been perhaps the most aggressive prospective candidate.
The ophthalmologist recently squeezed in a mission to perform eye surgeries in Guatemalaand invited news organizations to cover itbetween stops in Iowa and South Carolina. He's confirmed September appearances in California and Virginia, and October visits to North Carolina and New Hampshire, among dozens more possible stops.
The libertarian-leaning Paul, the son of former Texas Rep. Ron Paul, is trying to build on the small but passionate coalition assembled by his father. The elder Paul wasn't taken seriously by many Republicans, but Rand Paul has emerged as a leading GOP voice on foreign and domestic policy.
Working to move past a bridge-clogging scandal that shadowed his plans, New Jersey Gov. Chris Christie continues an aggressive travel schedule this fall as chairman of the Republican Governors Association.
Having already visited New Hampshire, he's announced a trip to South Carolina, where he'll have a chance to test his message with more conservative voters. He's also planning trips to Ohio, Wisconsin and Florida.
Christie leads a delegation of New Jersey business and political leaders to Mexico in early September, a trip that gives him a chance to bolster his appeal with Latino voters and burnish his foreign policy chops. And at home, Christie will unveil a budget plan that is sure to draw fury from Democrats and union leaders.
Eyeing a second presidential bid, Texas Gov. Rick Perry was already facing challenges related to his disastrous 2012 campaign before his recent felony indictments.
His advisers suggest the charges could actually help his political prospects, and he has pressed ahead with visits to Iowa, Washington, D.C., New Hampshire, and more.
Perry heads to Iowa in early September shortly before a weeklong economic tour across Asia. He'll turn his attention to helping Republican governors win reelection when he returns.
The Texas governor will launch a European tour in October.
The possible GOP field also includes Florida Sen. Marco Rubio, Louisiana Gov. Bobby Jindal and Indiana Gov. Mike Pence. Wisconsin Gov. Scott Walker hopes to use his reelection test this fall as a springboard into 2016. Others must convince skeptical party leaders they have mainstream appeal a group that includes conservative firebrand Texas Sen. Ted Cruz and social conservative Rick Santorum.
AP writers Jill Colvin in New Jersey and Mike Mishak in Florida contributed to this report.