Obama has expanded his lead on Clinton in many national polls and gained ground on her in the next battleground of Pennsylvania ahead of Tuesday's vote, despite furors over his remarks on small-town residents and inflammatory comments by his former pastor.

Clinton's image appeared to take a heavier hit after wrongly claiming she faced sniper fire in Bosnia in 1996. A Washington Post poll this week found more Americans have an unfavorable impression of her than at any time since she entered the national limelight in 1992.

"It hasn't been a bed of roses for Obama. He's had some problems. But she is the one whose negatives are going up," said Phil Noble, head of the South Carolina New Democrats group and an Obama supporter.

Obama has a nearly unassailable lead on the New York senator in delegates to the August nominating convention and in popular votes won in the first three months of the primary battle.

Clinton hopes a big Pennsylvania win ignites a strong run through the final nine contests, fundamentally reordering the race and giving her fresh evidence to argue she is the strongest candidate to face Republican John McCain in November's presidential election.

But polls show Obama has whittled her once substantial double-digit lead in Pennsylvania to single digits. A Zogby poll on Friday put her lead at 4 points, a Rasmussen poll showed it at 3 points and a Los Angeles Times poll earlier this week had it at 5 points.

A narrow Clinton win probably would be enough to keep her in the race, but would not stem another round of calls among Democrats for her to step aside and let Obama concentrate on the race with McCain.

OBAMA LEAD GROWS

Obama has expanded his national lead in several polls. A Reuters/Zogby poll released earlier this week put it at 13 points, and a daily Gallup tracking poll had it at 7 points, down from his high of 11 earlier in the week.

Obama continues to steadily win endorsements from superdelegates, the nearly 800 Democratic Party insiders who are free to back any candidate and who hold the key to winning the nomination.

"It doesn't seem like she has the power to alter the dynamic of the race anymore," said Simon Rosenberg, head of the Democratic advocacy group NDN.

Rosenberg said Clinton's scenario for winning the Democratic nomination was no longer believable.

"In every way you can measure it, he's won more delegates, he's won more states, he's raised more money, he has a better organization -- all the metrics one has of how to evaluate the race indicate he is winning and she is losing," he said.

Obama, an Illinois senator who would be the first black U.S. president, weathered a flap in March about the controversial comments of his former pastor, the Rev. Jeremiah Wright, by giving a widely praised speech on race.

He ignited another firestorm last weekend when his comments about small-town residents at a private San Francisco fundraiser became public. He said small-town residents were clinging to religion and guns out of bitterness about their economic struggles.

In the six-week lull between the last contest in Mississippi, which Obama won, and Tuesday's vote in Pennsylvania, those flaps and Clinton's Bosnia controversy dominated the campaign debate.

But so far, none of the controversies appears to have the strength to derail Obama.

"Voters in the end may not be that agitated about these kinds of things," said Linda Fowler, a political analyst at Dartmouth College in New Hampshire. "They really are more concerned about the war and the economy and which candidate is more effective."