He's running down McCain more often than the woman he's nominally still fighting for his own party's nomination. And he's running after white working-class voters, independents, Hispanics, Catholics and Jews - voting blocs that will be important in the November election and with whom he's had mixed successes.
Even as Obama tries to fight offin the few remaining primaries, he is campaigning in states that have already held elections and vowing to return to states where he lost to Clinton. His campaign has sent teams into battleground states, set up a program for signing up millions of Democrats over the next six months and is developing ads to use against McCain.
History shows that the earlier a candidate nails down the party nomination, the better his chances of winning. Obama did not have the luxury of an early win, so he did the next best thing. His team is seeking to tether McCain to President Bush, emphasizing McCain's support for the Iraq war and for renewing Bush tax cuts.
"Obviously, we don't want to wake up the morning after we become the nominee and not be prepared," said Obama campaign manager David Plouffe. Offering a campaign line Obama is already using, he said, "By November, every voter will know that McCain is offering a third Bush term."
Democratic strategists agree that Obama has his work cut out for him in defining himself on his own terms and countering assertions that he's inexperienced, elitist and out of step with the rest of the country.
"Partly what he's got to do is define his appeal to middle-class voters," said Democratic pollster Mark Mellman. "I don't think that will be hard. But it's something he has to do, provide some focus on their economic pain and on the issues that are animating him."
At first glance, the political landscape couldn't be better for the first-term Illinois senator, along with Democrats in general. Republicans are being pulled down by the unpopular Iraq war, the struggling economy and Bush's low approval ratings.
Yet McCain appears to have escaped much of the criticism directed at fellow Republicans. Polls show the Arizona senator to be competitive with Obama in a general-election matchup.
Obama's inability to win primaries in many big industrial battleground states, or to appeal to white working-class voters or to many older people, particularly women, make him vulnerable - as does his lack of economic and foreign-policy experience.
"Clearly Obama has to give people more confidence in his ability in protecting the country, where McCain has a huge advantage, wider than Bush enjoyed over John Kerry four years ago," said pollster Andrew Kohut, president of the independent Pew Research Center.
Most of those questioned in a Pew survey this month described McCain as "a centrist whose views are fairly close to their own." The same voters see Obama as far to the left of themselves.
Obama's team began a transition to general-election mode weeks ago.
He is reaching out to Hispanics, the nation's fastest-growing minority, and to Jewish voters. Both groups, while traditionally Democratic, eye him with some suspicion. And he's started to wear an American flag lapel pin.
Obama's team is trying to find ways to counter what former Bush speechwriter Michael Gerson calls "the Obama narrative," an effort by Republicans to portray Obama as a man of the academic left, out of touch with everyday American concerns.
Despite his roots as a child of a single mother who sometimes used food stamps, Republicans will remind voters of Obama's schooling at Columbia and Harvard Law, and his comments at a San Francisco fundraiser that those in small towns grow "bitter" and cling to religion and guns.
Obama also was hurt by the comments of his controversial former pastor, the Rev. Jeremiah Wright. The candidate first said he couldn't disown the pastor, but when Wright kept up his remarks and the political heat increased, Obama dismissed him as a "relic of an angry generation."
Obama has won much of his support from upscale voters, young people and blacks. But he's been having trouble connecting with white blue-collar workers.
A step in his outreach was the campaign's decision last month to hold more events portraying Obama among ordinary Americans - downing domestic beer in crowded taverns, eating breakfast at lunch counters, greeting factory workers, touring auto plants and visiting construction sites.
The tactic has proved more effective than the earlier focus on big rallies, said David Axelrod, Obama's chief strategist. "We're slow learners," he joked.
Primary election results from across the country have shown Obama has trouble winning support among Hispanics, too. While Hispanics traditionally vote Democratic, Republicans have been making inroads. Bush drew 44 percent of the Hispanic vote in 2004 - the highest of any Republican presidential candidate - and McCain is polling as high as 41 percent.
As for McCain, Obama's aides are developing ads intended to link the Arizona senator to Bush and chip away at his image as a maverick.
McCain's age may also work to Obama's benefit. McCain turns 72 in August and would be the oldest man to be elected to a first term as president. Some voters, Republicans and Democrats alike, express reluctance to vote for candidates over 70 years old, some surveys show. But advisers suggest Obama should be wary of approaching the topic directly, for fear of it backfiring.
Obama continues to face questions about his commitment to the Jewish community, another usually solid Democratic bloc. Some voters may be upset by his stated willingness to enter presidential-level talks with leaders of countries such as Iran and Syria.
"If you look at my writings and my history, my commitment to Israel and the Jewish people is more than skin-deep and it's more than political expediency," Obama told The Atlantic magazine.
Obama's next major decision will be selecting a running mate. Although some in both the Obama and Clinton camps have held out an Obama-Clinton combo as a "dream ticket," few close to either candidate expect to see it.
Among the speculation: If he wants to pick a woman, because of all the disappointed Clinton supporters, it might be someone like Kansas Gov. Kathleen Sebelius. If he wants to deal with McCain's advantage on national security, he might pick someone like former Sen. Sam Nunn of Georgia, New Mexico Gov. Bill Richardson or Sen. Jim Webb of Virginia.
And if he is looking for an Electoral College advantage, he might pick either Pennsylvania Gov. Ed Rendell or Ohio Gov. Ted Strickland, both of whom were Clinton supporters.
"I think the first important thing is for Obama and Mrs. Clinton to arrange a graceful exit for her and an endorsement for him by her with no conditions. It's essential that those who lost don't feel cheated," Georgetown University political scientist Stephen Wayne said.
Democratic strategist Doug Schoen suggests Obama should "think outside the box" and go for someone outside the party as his running mate, such as New York Mayor Michael Bloomberg, an independent, or Republican Nebraska Sen. Chuck Hagel, a Vietnam veteran.