Hillary Clinton must win Tuesday's Pennsylvania primary, but she needs to do more than simply scrape past rival Barack Obama to rescue her trailing White House bid.
The New York senator is 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 race to soothe party divisions.
She also 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 another seven voting showdowns stretching into early June.
A shock win by Obama meanwhile in the economically pummelled northeastern state would narrow Clinton's already unlikely route to the nomination, and likely snuff out her historic quest to be the first female president.
As it is, the former first lady position is perilous. She trails Obama in total nominating contests won, pledged delegates aportioned in those showdowns, the popular vote, and the crucial multi-million dollar campaign financing race.
But her camp stresses electability, not electoral mathematics and on Monday she said she would keep on fighting until the end of the marathon nominating battle.
"I'm going until everybody's had a chance to vote in this process," she said in a CNN interview."
Victory would bolster Clinton's claim that only she can solidify the Democratic powerbase, woo socially conservative working-class voters, and prevail in crucial presidential battlegrounds.
"Assuming we do well in Pennsylvania, it will again show that we are better positioned to win the big swing states that any Democrat needs to win in November," Clinton spokesman Howard Wolfson said Monday.
"The goal ... is to come out of Pennsylvania with a 'w' (win), and whether it's one vote or 1,000 votes or 10,000 votes, it does not matter."
Obama has splashed 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 in a San Francisco fundraiser that some smalltown 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.
Obama, who matched Clinton's intense sprint round the state's major media markets on Monday, however predicted in an interview with a Pittsburgh radio station that he would do surprisingly well.
"I am not predicting a win. I am predicting it is going to be close and we are going to do a lot better than people expect," he told KDKA.
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.
As the clock ticked down on the state campaign on Monday, Obama's camp accused Clinton of trying to scare Americans, after the release of a dark new campaign ad featuring Al-Qaeda kingpin Osama bin Laden.
Clinton played off the script of her ad during a rally in Pittsburgh.
"If you can't stand the heat, get out of the kitchen," Clinton said, using former US president Harry Truman's famous catchphrase.
"I am very comfortable in that kitchen making those decisions."
The 30-second broadcast does not mention Obama by name, but the Illinois senator's spokesman Bill Burton fired off a robust response and brought up Clinton's vote in 2002 to authorize the Iraq war, which his boss opposed.
"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.
"We already have a president who plays the politics of fear, and we don't need another."
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