Barack Obama said Monday he doesn't think he'll beat Hillary Clinton in Pennsylvania on Tuesday, but she might not get the win she needs.

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

Meanwhile, Clinton aides tried to downplay expectations, insisting they would be grateful for a single-digit win even though she started the race as much as 20 points ahead in some polls.

With less than 24 hours to go until the crucial Pennsylvania primary -- which could decide if Clinton's campaign goes on -- the two Democratic candidates kept up the barrage of negative rhetoric and advertising that they hoped would gain them high ground even as they stooped lower.

In the latest in a series of attacks, Clinton on Monday unveiled a new ad seeking to turn the corner on the "kitchen sink" metaphor being used by Obama to show Clinton's desperation.

With a video montage of the aftermath of Hurricane Katrina, home foreclosures and soldiers, the narrator says: "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's campaign countered quickly with a video clip of its own, but this one of Clinton's husband, former president Bill Clinton, during a 2004 campaign rally.

Bill Clinton, speaking to a crowd says: "Now, one of Clinton's Laws of Politics is this: If one candidate's trying to scare you and the other one's trying to get you to think, if one candidate's appealing to your fears and the other one's appealing to your hopes, you better vote for the person who wants you to think and hope."

The campaigns have been watching each other closely for cues, leaping at each other's words several times a day in the hours before Pennsylvania voters head to the polls.

It has been six weeks since any votes have been cast in the Democratic race and Clinton and Obama are in an all-out brawl as Clinton faces a test in which she must not only win Tuesday's vote, but continue to win by strong enough margins to regain the lead in the delegate count. Some observers say Clinton must win by double digits in Pennsylvania if she wants to have a chance at taking the nomination.

Clinton has 1,507 delegates to Obama's 1,645, with 2,025 needed to win. There are 158 pledged delegates at stake in Pennsylvania, and they will be divvied up proportionally between Clinton and Obama by the final vote tally. And out of the 23 Pennsylvania primaries held since the first one in 1912, 21 winners there went on to win the general election.

Polls are showing Clinton holding the edge. A Suffolk University poll released Monday (April 19-20, 600 Democrats, +/- 4 percent) showed Clinton ahead of Obama by 10 percentage points among Pennsylvania Democrats, 52 percent to 42 percent. And a new Quinnipiac University poll (April 18-20, 1,207 likely Pennsylvania Democratic voters, +/- 3.1) shows Clinton ahead 51 percent to 44 percent.

Clinton and Obama started anew Monday, with Obama scheduled for five campaign stops and Clinton scheduled for four. She also is dispatching Bill Clinton to two stops before the night is over.

Clinton's latest ad on Monday followed a skirmish that opened up Sunday over comments Obama made in Reading, Pa., where he told an audience that "either Democrat would be better than John McCain. ... And all three of us would be better than George Bush."

Seeing an opening, Clinton pounced on those words when she spoke later at a rally in Johnstown, Pa.

"Senator Obama said today that John McCain would be better for the country than George Bush," Clinton said. "Now, Senator McCain is a real American patriot who has served our country with distinction, but Senator McCain would follow the same failed policies that have been so wrong for our country the last seven years."

"We need a nominee who will take on John McCain, not cheer on John McCain. And I will be that nominee," Clinton said.

Late Sunday, Obama spokesman Tommy Vietor blasted back: For someone who agreed with John McCain on voting for war in Iraq, agreed with him on supporting trade agreements like NAFTA and special trade privileges for China, and agrees with him on taking money from Washington lobbyists, Hillary Clinton's latest attack is ridiculous even for her standards, and we're confident the people of Pennsylvania will see right through it."

And even though McCain is already the presumptive Republican nominee, his campaign jumped into the fray.

"The remark underscores that John McCain has the strength to change America and move this nation forward. Barack Obama is a new face who represents old ideas," said McCain spokesman Tucker Bounds.

Health care, the economy, honesty and the question over who has the least questionable associations highlighted the Democratic candidates' attacks on each other, and the two unveiled new advertisements aimed at crippling one another.

"She's taken more money from lobbyists and special interest than any candidate, Republican or Democrat. ... When you ask yourself why it is that we don't have health care that everyone can count on, think that drug and insurance companies have spent a billion dollars in the last 10 years in lobbying and contributions and PR, so it's surprising that laws that are passed in Washington are good for them, but not as good for you," Obama said in Reading.

In Bethlehem, Pa., Clinton reminded her audience of the stakes later this week, and chided Obama for negative remarks.

"This week, we had a debate and it showed you the choice you have. And it's no wonder that my opponent has been so negative these last few days of the campaign. Because I think you saw a big -- you saw a big difference between us. It's really a choice of leadership," Clinton said. "I'm offering leadership you can count on.

"The two also blanketed the state with attack ads."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 new Clinton commercial airing in the campaign's final days.

If anything, Obama upped the ante with his rebuttal. His ad said he "doesn't take money from special interest PACs or Washington lobbyists -- not one dime." Clinton does, it added, and accused her of "eleventh-hour smears paid for by lobbyist money."

Clinton also has sought to paint Obama as elitist following remarks he made about small town voters, while Obama's campaign has jumped on a recent poll showing new erosion among voters' belief in Clinton's honesty. Obama also has been forced to distance himself from his bombastic former pastor, and Clinton had to admit misspeaking about a trip to Kosovo in her husband's administration.

In the meantime, campaign surrogates handicapped their candidates' chances on the Sunday talk circles. Clinton supporter Sen. Charles Schumer, D-N.Y., says the race isn't a lock, but Pennsylvania and the next-biggest remaining prize, Indiana, are in Clinton's sights.

"I think that she's going to do very well in Pennsylvania, and I think she's going to better than either the polls, which have always underestimated her, or the pundits have stated," Schumer said on "FOX News Sunday."

Predicting a win by a "significant amount" in Pennsylvania, he added: "I think she's going to win in Indiana. And I think that she has momentum, and you're going to see people saying Hillary Clinton is the best candidate to both beat John McCain."

Clinton's newly appointed chief strategist Geoff Garin suggested she will continue her campaign no matter what the outcome in Pennsylvania.

"We're going to let the process play through," Garin said during an interview Sunday on NBC's Meet the Press.

And though he acknowledged that Clinton will have to "do well" in the remaining contests, Garin stressed that "neither candidate will have enough pledged delegates when the last votes are cast on June 3." He also advised that unpledged superdelegates wait until the last contest to cast their votes.

But, also on "FOX News Sunday," Sen. Dick Durbin, D-Ill. -- an Obama supporter -- said the odds are stacked against Clinton."

The math is very unforgiving at this point when it comes to delegate counts, and that's what it's all about," Durbin said.

"Senator Clinton needs more than 60 percent of the vote in Pennsylvania. ... If you look at the remaining contests, you understand that the Clinton campaign is running out of real estate. There are only a handful of states left. She needs over 60 percent of the vote in each one of them to catch up with Barack Obama," Durbin said.

But the question of whether Clinton should step aside should be up to her, and her alone, said Obama's chief campaign strategist David Axelrod, who appeared alongside Garin."

I don't believe anyone should tell Senator Clinton to get out of the race," said Axelrod, who criticized Clinton on a wide range of issues.The two campaigns are also attacking each other on the airwaves, although Obama has been outspending his rival about 2-to-1.

The Clinton campaign was sore after a new Obama ad released this weekend that criticized Clinton's health care proposal because, Obama's ad said, Clinton's plan mandates people to have health insurance, and would penalize those who do not.

Clinton responded Saturday: "I just heard that my opponent has put up an ad attacking my health care plan, which is kind of curious, because my plan covers of everybody, and his plan leaves out 15 million. ... Now instead of attacking the problem, he chooses to attack my solution.

Obama Saturday night in Lancaster focused on on the subject of NAFTA, saying Clinton supported it during her husband's administration, but then has come out against it during the campaign. "She can talk about supporting NAFTA when ... her husband's president. And she says, you know, she's out there campaigning for it, and then suddenly she's running for president and says she opposes it," Obama said.

FOX News' Bonney Kapp and Aaron Bruns and The Associated Press contributed to this report.