Barack Obama says his rival Hillary Clinton will likely win Tuesday's Pennsylvania primary, the largest of the last few contests in the race to become the Democratic presidential candidate, but not by much.

"I'm not predicting a win," Obama told Pittsburgh radio station KDKA on Monday. "I'm predicting it's going to be close and that we are going to do a lot better than people expect."

Several polls suggest Clinton has maintained a slight edge over the Illinois senator after weeks of campaigning in Pennsylvania, familiar turf for Clinton, whose father's family comes from the northeastern city of Scranton.

With critics calling for Clinton to drop out of the race so Democrats can focus on beating Republican John McCain in this fall's election, the New York senator badly needs to win this state, the biggest of the last 10 contests.

Obama is leading in the number of states won, delegates gained and popular vote, while Clinton's campaign is mired in financial woes.

Clinton was $10.3 million in debt at the start of April, with only about $9 million cash on hand for the primaries. Obama reported having $42 million.

Clinton still hopes to woo the so-called superdelegates, elected legislators and party insiders who can vote as they wish, by convincing them she would have a better chance of winning large states full of blue-collar workers, such as Pennsylvania, in November's election.

In the last major votes in Texas and Ohio six weeks ago, Clinton pulled off victories that kept her in the race. Pennsylvania is also expected to favour Clinton, with the state's large proportion of older, white, working-class voters.

Recent statements by Obama may have hurt him with those voters.

He took heat over suggestions that small-town Americans "cling to guns or religion or antipathy to people who aren't like them" because they are "bitter" about being plunged into a recession.

New ad suggests Obama can't handle crises

Clinton's biggest blunder as of late was her claim that she'd braved sniper fire as the first lady when her plane landed in Bosnia. She later admitted she'd made a blunder when videotape showed the incident hadn't happened.

On Monday, the Clinton campaign released a new television ad with images of Osama bin Laden, Hurricane Katrina and historic events such as the stock market crash, suggesting Obama is not up to the task of handling such major crises.

"Who do you think has what it takes?" asks an announcer.

"It's the toughest job in the world. You need to be ready for anything — especially now, with two wars, oil prices skyrocketing and an economy in crisis. Harry Truman said it best — if you can't stand the heat, get out of the kitchen."

Obama spokesman Bill Burton attacked Clinton for pandering to the "politics of fear" and said she's to blame for allowing bin Laden to escape by supporting the war in Iraq and diverting the U.S. military.

"It's ironic that she would borrow the president's tactics in her own campaign and invoke bin Laden to score political points," Burton said.

With files from the Associated Press