WASHINGTON — After weeks of sniping at each other from a distance, Hillary Clinton and Barack Obama meet face to face tonight in what could be their last debate.

The showdown, at the National Constitution Center in Philadelphia, will be televised from 8 to 9:30 EDT on ABC. ABC News anchors Charles Gibson and George Stephanopoulos will moderate.

Tonight's clash comes at a crucial time, with Obama still the front-runner for the Democratic presidential nomination but trailing Clinton in opinion polls in Pennsylvania, which holds its primary next Tuesday. Obama also is facing new questions about how thoroughly he's been vetted and the debate is likely to include some of the flash points and revelations that have dominated the increasingly nasty campaign in recent weeks.

Among them: Obama's remarks that working-class Pennsylvanians were clinging to religion and guns out of bitterness about their economic standing, his relationship with a pastor seen in videotapes damning the United States and Clinton's efforts to capitalize on both to revive her campaign.

After gaining on Clinton in Pennsylvania, where a come-from-behind win would create enormous pressure on her to quit the race, polls suggest that Obama has stalled there.

"There is pressure on him to rebound," said Larry Rasky, a veteran Democratic strategist.

Obama's key challenge tonight is to connect with working-class voters, particularly white working-class voters.

He's long had a hard time winning them away from Clinton, and he's been newly hammered for his comments that critics charged linked such voters' embrace of religion and guns to such negative values as xenophobia.

While Obama tried photo ops such as bowling — Clinton tried a shot and a beer — he needs to make a more substantive connection with a populist economic message, more John Edwards than Adlai Stevenson, Rasky argued.

"He needs to step up and show he's on their side," Rasky said.

A new poll of likely primary voters by Quinnipiac University found Clinton stopping Obama's gains on her and holding on to a lead of 50-44 percent, but it also indicated that she hadn't gained anything from her effort to portray Obama as an out-of-touch elitist. The poll of 2,103 likely voters conducted over the weekend had a margin of error of plus or minus 2.1 percentage points.

"She seems to have halted the erosion of whites and white women in particular," said Clay Richards, the assistant director of the Polling Institute at the Connecticut university.

"She even gained back some ground in the Philadelphia suburbs, the area where elections are won and lost in the Keystone State."

Watch for both candidates to be asked about guns, an issue they've largely avoided so far.

Democrats generally have shied away from the issue of gun control for several years, seeing it as a political loser. Clinton and Obama also have shied away from talking about a big Supreme Court case on gun control in Washington, D.C.

The issue could be even riskier in Pennsylvania, home to nearly 1 million licensed hunters. The politically active National Rifle Association has 250,000 members in Pennsylvania, more than in any other state.

But Obama's remarks could force the issue into the debate, especially on the one-year anniversary of the massacre at Virginia Tech university and in a city whose violence has earned it the unflattering nickname "Killadelphia."

Clinton faces challenges as well.

First, the New York senator has to press her case against Obama without inviting a backlash from Democrats weary of fighting. She was hissed at in Pittsburgh this week when she ripped the Illinois senator, though it isn't known whether that came from her supporters, neutral Democrats or Obama supporters.

Also, she'll probably be pressed to explain her story about landing in Bosnia under sniper fire, which proved to be untrue. She said she misspoke. Her husband said last week that she'd said it one time late at night, though McClatchy reported that she'd said it repeatedly, day and night.

It will be the 21st time that Clinton and Obama have debated over the yearlong campaign but the first time they've faced each other since a debate Feb. 26 in Cleveland.

It also could be the last.

While Clinton — still the underdog for the nomination — readily accepted another debate proposed for April 27 in North Carolina, Obama hasn't yet committed. Aides say that he wants to wait to see what happens with the Pennsylvania primary next week before agreeing.

Obama's controversial comment, from the Web site Huffington Post:

"You go into some of these small towns in Pennsylvania, and like a lot of small towns in the Midwest, the jobs have been gone now for 25 years and nothing's replaced them. And they fell through the Clinton administration, and the Bush administration, and each successive administration has said that somehow these communities are gonna regenerate and they have not. And it's not surprising then they get bitter, they cling to guns or religion or antipathy to people who aren't like them or anti-immigrant sentiment or anti-trade sentiment as a way to explain their frustrations."