PHILADELPHIA - Hillary Rodham Clinton and Barack Obama collided Tuesday in the Pennsylvania primary, the last of the big-state contests in a Democratic presidential campaign growing more negative the longer it goes.
With 158 delegates at stake, Pennsylvania offered the largest prize remaining in a primary season that ends on June 3.
Obama began the night with a delegate lead, 1648.5-1509.5, out of 2,025 needed to win the nomination.
Both rivals sought to shape expectations in advance. Obama said he expected to lose, but narrowly, and worked to limit any gains Clinton made in the delegate chase.
"We think we've made enormous progress" though "it's an uphill battle," Obama said as he greeted patrons at a Pittsburgh diner Tuesday. He noted that polls show a tighter race than just a few weeks ago, but said: "We still, I think, have to consider ourselves the underdog."
"We've got a great organization. A lot of it's going to depended on turnout," Obama added.
For her part, Clinton dismissed the notion that she needed a blowout victory to quell doubts about her candidacy.
"I think a win is a win. Maybe I'm old fashioned about that," she told reporters. "I think maybe the question ought to be, why can't he close the deal with his extraordinary financial advantage? Why can't he win a state like this one if that's the way it turns out ... big states, states that Democrats have to win."
The former first lady greeted voters at a polling place in Conshohocken, in suburban Philadelphia. She also visited a local restaurant there and picked up a Philly cheese steak.
Clinton's aides disputed suggestions she would prevail by a double-digit margin.
Beyond that, a defeat for Clinton could spell the end of her candidacy. But a sizable win would strengthen her claim to being the stronger general election opponent, an argument she has made to superdelegates who hold the balance of power at the party convention in Denver in August.
Whatever the outcome, the six-week run-up to the primary was notable for close-to-the-ground campaigning normally reserved for the early-voting states of Iowa and New Hampshire, and for the decidedly negative tone of its final few days.
Philly cheesesteaks were commonplace. Obama went bowling as he reached out for the support of working-class voters, and Clinton showed her blue-collar bona fides with a shot of Crown Royal and a beer chaser.
Flush with cash, Obama reported spending $11.2 million on television in the state, compared with $4.8 for Clinton.
The tone of the campaign was increasingly personal.
"In the last 10 years Barack Obama has taken almost $2 million from lobbyists, corporations and PACs. The head of his New Hampshire campaign is a drug company lobbyist, in Indiana an energy lobbyist, a casino lobbyist in Nevada," said a Clinton commercial that aired in the final days of the race.
Obama responded with an ad that accused Clinton of "eleventh-hour smears paid for by lobbyist money." It said that unlike his rival, he "doesn't take money from special interest PACs or Washington lobbyists — not one dime."
To the delight of Republicans, the six-week layoff between primaries produced a string of troubles for the Democrats.
Obama was forced onto the defensive by incendiary comments by his pastor, Rev. Jeremiah Wright, then triggered controversy on his own by saying small-town Americans cling to their guns and religion because of their economic hardships.
Clinton conceded that she had not landed under sniper fire in Bosnia while first lady, even though she said several times that she had. And she replaced her chief strategist, Mark Penn, after he met with officials of the Colombian government seeking passage of a free trade agreement that she opposes.
McCain, the Republican nomination already his, rose in the polls as he prepared for the fall campaign.
The remaining Democratic contests are primaries in North Carolina, Indiana, Oregon, Kentucky, West Virginia, Montana, South Dakota and Puerto Rico, and caucuses in Guam.