Barack Obama and Hillary Rodham Clinton hit each other with TV attack ads in the final days before the critical Pennsylvania primary, notching up the animosity in the Democratic presidential campaign on the eve of Wednesday's nationally televised debate.

With Clinton trailing in delegates and 11 points behind in a new national poll, she faces Obama in Philadelphia on Wednesday for what could be the climax of an extremely contentious few days.

The former first lady has labeled Obama an elitist for remarks he made about bitterness among economically hard-pressed working class voters. The first-term Illinois senator has countered with charges that Clinton was pandering by drinking a shot of whiskey in front of TV cameras and with stories of learning to shoot a gun at her father's knee.

But, in campaign appearances Tuesday, the candidates sidestepped opportunities to deepen their conflict, leaving the tough talk to their advertisements.

Clinton's new ad pulled together a string of sound bites from Pennsylvania voters who chastised Obama for his remarks that Pennsylvania's working class was clinging to guns and religion in bitterness over job losses and falling living standards.

Obama's ad shows Clinton being briefly heckled as she criticized Obama at a Monday gathering of the Alliance for American Manufacturing. The ad concludes with Obama accusing Clinton of playing the "politics of division and distraction."

The heat in the Democratic race reflects the view that Clinton must score a sizable victory in the Pennsylvania primary on Tuesday to keep her candidacy alive. She trails Obama 1,640-1,504 in delegates with only 10 state primary and caucus votes remaining after Pennsylvania, the largest state still to vote.

Because Democratic state contests are not winner-take-all in terms of delegates, it is statistically extremely unlikely for Clinton to overcome Obama's lead in the so-called pledged delegate count.

Neither candidate will be able to clinch the 2,025 delegates needed to win the nomination without the approval of the party's nearly 800 superdelegates. Of that group, 254 have said they back Clinton, and 226 are supporting Obama. His overall delegate lead includes the committed superdelegates.

Clinton faced further challenges as a new poll showed that more Americans have an unfavorable view of her than at any time since 1992.

Fifty-four percent said they have an unfavorable view of Clinton, up from 40 percent after she won the New Hampshire primary in early January, according to The Washington Post-ABC News poll. The survey, which has a margin of error of plus or minus three percentage points, found Obama's favorability ratings also had dropped, but remained more positive than negative.

In Pennsylvania on Tuesday, Obama dismissed a voter's suggestion that Clinton, when she called him elitist, had "bordered on (calling him) uppity."

"It's politics," the Obama told a town-hall meeting on veterans affairs. "This is what we do politically, when we start getting behind in races. We start going on the attack."

Clinton, meanwhile, was jolted with a fresh reminder that party elders have no appetite for a campaign that drags into the convention in late August. Rep. Barney Frank, a Massachusetts Democrat, said the candidate who trails in the delegate chase should quit by June 3. "Probably sooner," he told The Associated Press in an interview.

Frank's remarks carried extra weight because of his long-standing support for Clinton and his status as a superdelegate.

Clinton became the third of the three remaining presidential hopefuls to appear before newspaper editors at their convention, following appearances by Obama and Republican Sen. John McCain on Monday.

She accused President George W. Bush of having expanded executive power to the detriment of the Constitution, while often operating in secrecy.

"I'll end the use of signing statements to rewrite the laws Congress has passed. I'll shut down Guantanamo (the American military prison in Cuba), disavow torture, and restore the right of habeas corpus," she said.

"And I'll end the practice of using executive privilege as a shield against the public's right to know and Congress's duty to oversee the president."

Clinton's agenda for her first 100 days in office included the start of a troop withdrawal from Iraq and submitting a budget to Congress that rolls back some of Bush's tax cuts. She also promised to sign bills he has vetoed to expand federal embryonic stem-cell research and broaden government-supported health care to millions of lower-income children who now go without.

For his part, McCain called for a summer-long suspension of the federal gasoline tax and several tax cuts as the likely presidential nominee sought to stem the public's pain from a troubled economy.

Timed for the day millions of Americans filed their income tax returns, McCain offered some immediate steps as well as long-term proposals in a broad economic speech. The nation's financial woes have replaced the Iraq war as the top concern for voters, and McCain, who has said economics is not his strongest suit, felt compelled to address the problems as he looks ahead to the November general election.

"In so many ways, we need to make a clean break from the worst excesses of both political parties," McCain told an audience at Carnegie Mellon University. "Somewhere along the way, too many Republicans in Congress became indistinguishable from the big-spending Democrats they used to oppose."

The Republican fended off criticism on his Iraq policies, after Senate Foreign Relations Committee Chairman Joe Biden said Tuesday that McCain would continue Bush's practice of pursuing the war in Iraq at the expense of other urgent global issues.

"When it comes to Iraq, there will be no change with a McCain administration ... and so there is a real and profound choice for Americans in November," Biden said in a speech at Georgetown University.

McCain, the certain Republican presidential nominee, backs Bush's policy in Iraq and favored last year's increase in U.S. troops there.

Asked whether Biden was right, McCain argued anew that he spent four years before the troop influx pressing for more forces, and he said the right strategy is now in place.

"The present strategy is succeeding in Iraq and I respect the view of those who don't agree," McCain told reporters in Pennsylvania.

Biden dropped out of the Democratic presidential race after Iowa's Jan. 3 caucuses. He has not taken sides in the contest between Obama and Clinton.