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.