With the important Pennsylvania contest six weeks away - a near eternity in presidential primaries - Obama is playing down his chances here, even though a victory would effectively finish Clinton. His aides are emphasizing instead the need to campaign in North Carolina, Indiana and other presumably friendlier states that will vote even later.
Clinton, meanwhile, is banking heavily on Pennsylvania. A solid win here could sustain her claim to late-season momentum and the ability to win big, industrial states.
The New York senator traveled to Pennsylvania immediately after the March 4 primaries, when her Ohio and Texas wins kept her campaign alive. Obama did not go until Tuesday, a full week later. He held one public event, north of Philadelphia, before returning to Chicago for a day.
Obama's revised strategy is, essentially, a mathematical calculation. If Clinton wins a few more delegates than he does in Pennsylvania, Obama figures, he can offset them in the nine states and territories scheduled to vote later.
His current lead of roughly 100 delegates would stay about the same, the thinking goes. That would position him to tell the all-important superdelegates this summer there is no justification for them to tip the nomination to Clinton.
Superdelegates are party officials who can side with any candidate. They will decide the nomination because neither Obama nor Clinton can secure enough pledged delegates through voting rules that allocate state delegates on a proportionate, not winner-take-all, basis.
The strategy may be politically sound. But it lacks the swagger and self-confidence that Obama carried out of Iowa, into South Carolina and beyond.
At the time, he explained his breakthrough wins in Iowa and South Carolina - and his momentum-stopping loss in New Hampshire - largely as matters of time. He and his rivals had weeks to campaign in Iowa, then five days for the final push in New Hampshire, and then 18 days before the South Carolina contest.
Obama prevailed in Iowa and South Carolina, he said, largely because he could devote at least two weeks to campaigning county by county.
"What we have seen in the first four contests is that the more people know me, the better I do," Obama told reporters shortly after his South Carolina win. "In Iowa, we had huge amounts of time, and we were able to win across the board. In these other early states like South Carolina, people have had a lot of interaction with our organizers."
With 42 days left before the April 22 Pennsylvania primary, Obama has time to conduct such saturation campaigning. Clinton must win Pennsylvania to make her case to superdelegates.
Rather than go for the knockout punch, however, the Obama campaign is playing down expectations, emphasizing the need to keep campaigning in other states, and portraying Pennsylvania as clearly favorable to Clinton.
"It's an uphill fight for us," David Axelrod, Obama's chief media strategist, said in an interview. "It's a mistake to reduce the contest to one state."
Indeed, Clinton has clear advantages here. Her father, Hugh Rodham, grew up in Scranton and is buried there. The former first lady visited Scranton many times as a child and still has relatives in Pennsylvania. Her campaign plans to have 250 staffers and at least 23 offices in the state.
Pennsylvania bears many demographic similarities to Ohio, which Clinton won by 10 percentage points. It is home to many older voters and white blue-collar workers with little or no college, two staples of her base.
Clinton also has the support of Ed Rendell, the state's popular, garrulous Democratic governor, and he appears ready to campaign hard for her, as Gov. Ted Strickland did in Ohio.
When Obama visited a wind turbine production plant north of Philadelphia on Tuesday, he acknowledged not being in the state for "a long time." But in the coming weeks, he said, "I'll be talking with folks all across this state from Pittsburgh to Harrisburg, from Philadelphia to Wilkes-Barre."
Clinton will spend the bulk of her time in Pennsylvania but also will visit other key states. Advisers argue that her prospects are strong in West Virginia, whose primary is May 13, and in Kentucky and Oregon, which vote May 20. All three have a healthy share of older, blue-collar voters.
The campaign expects Indiana, whose primary is May 6, to be a battleground because of its large black population and the fact that the northern part of the state is in the Chicago media market, where Obama is often on TV and in the news. But the state also has a significant blue-collar population and Evan Bayh, the state's senator and former governor, will actively campaign for her.
North Carolina's primary is also May 6, and Clinton officials expect Obama to win there because most of the state's Democratic voters are either black or very liberal. But they say she will compete there anyway to pick off delegates in congressional districts in the eastern and western parts of the state.
The Clinton campaign expects that she will do well in Puerto Rico's June 1 primary because of her strength among Hispanic voters. Her advisers give the edge to Obama in Montana and South Dakota on June 3.