The Obama campaign lists several factors working against it in Pennsylvania, the sixth-most populous U.S state, with about 12.3 million residents.
Clinton, a New York senator, has the support of some prominent Democratic politicians, including popular Gov. Ed Rendell, who controls a powerful party machine.
Obama got the backing last week of Pennsylvania Democratic Sen. Robert Casey, but aides said his endorsement, while important, could not offset the impact of Rendell's support for Clinton.
Clinton also has family roots in the Northeastern state, which she visited regularly as first lady when her husband, Bill Clinton, was president, and more recently. By contrast, Obama came to the state just four times between the Texas and Ohio primaries on March 4 and the start of his tour last Friday.
In those primaries, perceptions that Obama could win hurt his campaign when Clinton scored a big victory in Ohio and gained more votes in the Texas primary. As a result, Clinton's aides were able to argue Obama struggled in large states.
Crucially, the demographics of Pennsylvania appear to favour Clinton, who outscored him by more than 10 points in neighbouring Ohio, which has similar concentrations of older, working-class voters with strong union ties.
Many of Pennsylvania's voters are conservative Democrats who favour hunting and oppose abortion but see a positive role for government in redressing economic imbalances.
EXPECTATIONS MATTER
If his campaigning further narrows the gap in the polls, expectations of an upset victory will grow and a loss would then be doubly painful.
"If he doesn't win, he doesn't want it written that he put in maximum effort, and if he gets it into single digits he claims victory and says, 'Yes, I can do well with these voters," said Terry Madonna, professor of public affairs at Franklin and Marshall College in Lancaster, Pennsylvania.
He said Obama, to do well, needed to score big in the largest city, Philadelphia, and its suburbs, where blacks make up a high proportion of Democrats, and must limit losses elsewhere.
Like Clinton, Obama has taken cues from the populist rhetoric of former Democratic presidential candidate John Edwards, as he tries to court the state's many blue-collar workers.
In a fiery speech in Philadelphia on Wednesday, he expressed "outrage" over multimillion-dollar bonuses given to top executives of Countrywide Financial, a company at the centre of the housing crisis, as the firm was being sold.
He also took a swipe at Clinton, poking fun at her comparison of herself with the movie character Rocky Balboa in an attempt to portray herself as the underdog in the race.
"Last time I checked, I was the underdog in this state," Obama said.
