Pennsylvania Democrats on Tuesday cast judgement on Hillary Clinton and Barack Obama in a rancorous primary the former first lady must win well to steady her wobbling White House quest.

The New York senator was tipped for victory in late opinion polls, but many observers think it will take a double-digit triumph to stave off more calls for her to quit the epic Democratic nomination race.

But she told reporters in the Philadelphia suburb of Conshohocken that a simple victory would be sufficient.

"I think a win under any circumstances is a terrific accomplishment," she said after greeting cheering supporters at a polling station who chanted "madame President."

"Obviously, there is no guarantee that we know what is going to happen."

Obama downplayed the likelihood that he could win in Pennsylvania, a victory that would likely knock Clinton out of the race, but pointed out that he had whittled down her lead in most polls from 20 points to single figures.

"I always think we had a chance," he told reporters in Pittsburgh, western Pennsylvania, saying the race would be decided by which candidate was best able to turn out their core supporters.

Clinton needs a fresh burst of momentum ahead of the next round of contests in Indiana and North Carolina on May 6, which are followed quickly by the last six voting showdowns of the epic nominating battle into early June.

She played up Obama's significant fundraising edge, which has allowed him to triple her advertising buy in the northeastern state, according to her campaign's estimates.

"Maybe the question ought to be, why can't he close the deal with his extraordinary financial advantage, why can't he win a state like this one if that is the way it turns out?"

Polls opened in the economically pummelled northeastern state at 7:00 am (1100 GMT) and close at 8:00 pm (0000 GMT).

Clinton's position is already perilous.

She trails Obama in total nominating contests won, pledged delegates apportioned in those showdowns, the popular vote, and the crucial multi-million-dollar campaign financing race.

But her camp stresses electability in the November general election, not primary electoral mathematics.

Victory here would bolster Clinton's claim that only she can solidify the Democratic power base, woo socially conservative working-class voters, and prevail in crucial presidential battlegrounds.

Obama, who would be the first African-American US president, has spent millions of dollars in advertising in the state, and sharpened his attacks on Clinton, in an apparent bid to land a knockout that would allow him to set his sights on Republican John McCain.

But his remarks at a San Francisco fundraiser that some small town Americans were "bitter" over the economic squeeze, and so clung to religion and guns, may have dampened his poll surge, which saw him cut Clinton's lead to a few points.

Latest opinion surveys appeared to show Clinton headed for victory, but her hopes of a campaign-altering blowout seemed uncertain.

She led Obama 52 percent to 42 percent in a Suffolk University survey. A Quinnipiac University poll had her up seven points, 51-44 percent.

Clinton is making her fervent case to Democratic "superdelegates," the party officials who will now effectively crown the nominee, since neither candidate is likely to reach the 2,025 pledged delegates needed to win outright.

Obama's camp on Monday accused Clinton of trying to scare Americans, after the release of a dark new campaign ad featuring Al-Qaeda kingpin Osama bin Laden, which implied her rival was too inexperienced to serve as president.

Clinton kept up the crisis talk on Tuesday, threatening to "obliterate" Iran if it launches a nuclear attack on Israel, in an interview with ABC News.

Even a 10-point victory for Clinton in Pennsylvania would not do much to cut into her rival's delegate lead, as the state's 158 delegates will be doled out under the Democratic system of proportional representation.

Obama currently leads by 1,650 total delegates to Clinton's 1,508, according to independent website RealClearPolitics.com