Barack Obama tried his hand at bowling, bottle-fed a calf at a dairy farm and toured a chocolate factory as he sought to connect with voters in Pennsylvania, a crucial state in the fight for the Democratic U.S. presidential nomination.

Ending a six-day bus tour on Wednesday in a state where his rival Hillary Clinton is heavily favoured, the Illinois senator hoped to erode her advantages, especially with white working-class voters who have been slower to warm up to him than the young people and more affluent voters who have flocked to his rallies.

The Columbia University and Harvard-educated Obama, known for his sweeping oratory, sought to show a more down-to-earth side of himself, talking of his upbringing by a single mother and his early career as a community organizer in Chicago helping laid-off steelworkers.

Obama, who would be the first black president, leads in a tight national race with Clinton, who would be the first woman to win the White House, for the right to represent the party against presumed Republican nominee John McCain in the November presidential election.

But publicly, at least, he's not raising expectations of a win in the state's April 22 nominating contest. Most recent polls have put him behind Clinton, although the race has tightened.

"We are the underdog in Pennsylvania," Obama told voters in Johnstown. "We may not be able to win."

Obama visited a steel mill in Pittsburgh, a wire factory in Johnstown, a restaurant, and a bowling alley in Altoona - in a reprise of the small-scale, person-to-person campaigning used in states that voted earlier, such as Iowa.

At a meeting with voters in Johnstown, Doug and Trish Crump, both Democrats, did not applaud as Obama ran through the reasons why voters should choose him over Clinton.

"He is very charismatic and you automatically look to see what's the substance behind that," said Trish Crump, 48.

Doug Crump, 51, a clergyman, said he "cringed" as Obama said he would reconfigure U.S. foreign policy and avoid the "politics of fear," an allusion to charges that Republicans have used fear of another September 11-type attack on the United States as a way to scare up votes.

Retired railroad worker Tim Anderson, 53, who shook hands with Obama at an Italian marketplace in Philadelphia, said he would vote for Obama because he considered him "the lesser of two evils." He said he distrusted Clinton and liked Obama, although he had misgivings about controversial comments in the past by Obama's long-time pastor, the Rev. Jeremiah Wright.

"I'm a little up in the air about what happened with his pastor," he said of Wright. "That guy's a bigot."

DISADVANTAGES

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.