The last few weeks have produced a drumbeat of trouble for the Democratic front-runner. A decisive Pennsylvania loss to Hillary Rodham Clinton, the dust-ups over his former pastor and remarks about "bitter" small-town voters – all appear to have damaged his standing in the polls, both nationally and in the states voting Tuesday.
Democratic voters now see him in a much closer nomination fight with Senator Clinton, according to recent polls. And while the Indiana race had always been tight, Senator Obama has watched his once- secure double-digit lead in North Carolina dip to three percentage points.
A dual victory Tuesday stands as his best chance to shut down the prolonged nomination fight, analysts say. But if he loses Indiana and if North Carolina is close, Clinton can continue to argue her candidacy to the only group numerous enough to save it: the nearly 800 elected officials and party leaders called superdelegates, some 274 of whom remain undecided.
Obama leads Clinton 1,744 to 1,608 in the race for the 2,025 delegates – pledged and super – required for the nomination, according to an Associated Press count on Monday.
"Hillary is looking for evidence to lead the superdelegates to look past the numerical advantage that Barack Obama has and will have," says Lawrence Jacobs, director of the Center for the Study of Politics and Governance, at the University of Minnesota. "A win in Indiana and/or a win or close second in North Carolina will put wind in those sails."
Clinton needs tailwinds now more than ever. Over the past week, even as his poll numbers dropped, Obama has picked up superdelegates at a faster clip than the New York senator. Among them were two former chairmen of the Democratic National Committee, including Joe Andrew, the party's leader under Bill Clinton and a former supporter of Hillary Clinton.
But Clinton aides argue that the new poll numbers reflect a shifting landscape.
Geoff Garin, Clinton's chief strategist, told reporters Thursday that there had been "a real change of the dynamic in terms of the candidate who is consistently [viewed as] strongest against" the presumptive Republican nominee, Sen. John McCain.
At a tractor dealership in Kinston, N.C., Friday, Clinton billed the contests Tuesday as nothing short of a "game changer," a sign of growing confidence in the face of Obama's recent trials.
Seventy-two pledged delegates are up for grabs in Indiana, and 115 in North Carolina. Because both contests are expected to be close and Democrats divide a state's delegates according to the popular vote, the primaries Tuesday will not significantly alter Obama's lead in pledged delegates.
Indiana is a staunchly Republican state almost certain to remain in the GOP column in November. But a loss there Tuesday after his defeat in Pennsylvania would keep alive questions about the Illinois senator's standing with white working-class voters, a key bloc in the general election.
Obama's decline in the polls in recent weeks – he is now running even with or behind Clinton in some national surveys of Democratic voters – in large measure is due to a slide among those voters.
According to a Pew Research Center survey released Thursday, Clinton's lead over Obama among whites without a college degree rose to 40 percentage points, from 10 in late March. Her lead among whites with incomes below $50,000 rose to 24 percentage points, from two.
"The tightening Democratic race reflects a modest but consistent decline in Obama's personal image rather than improved impressions of Clinton," the authors of the Pew study wrote.
Not coincidentally, both Clinton and Obama have trained their messages in recent days on gasoline prices, factory closures, and other economic issues resonant with blue-collar voters.
Clinton chatted with a sheet-metal worker pumping gas in South Bend, Ind., last week – as news cameras clicked – and then recoiled at the price. "Sixty-three dollars for just about half a tank," she exclaimed, according to an Associated Press account.
The contests will also be in part a referendum on a pocketbook issue that has drawn a rare bright line between the candidates: a summer holiday on the 18.4 cents-a-gallon federal gasoline tax.
Clinton and Senator McCain say the proposal would help Americans through lean times, while Obama has labeled it a political "gimmick."
"It's not an idea to get you through the summer," he said last week. "It's an idea to get them through an election."
Clinton still leads Obama among superdelegates, 269 to 252, according to a tally by the independent website RealClearPolitics.com.
But Obama has halved her lead over the past two months, and won a raft of new superdelegates after his public break last Tuesday with his controversial former pastor, the Rev. Jeremiah Wright Jr.
The primaries Tuesday will be a measure of his success putting the Wright episode behind him with voters, says Ronald Walters, a professor at the University of Maryland in College Park and author of "Freedom is not Enough: Black Voters, Black Candidates, and American Presidential Politics."
"It will test whether this is an issue they really care about," he says. "It looks to whether or not he can recover."