SIOUX FALLS, S.D.  In his telling, his opponent is wrong on the Iraq war and wrong on the ailing economy, a would-be George W. Bush running for what amounts to a third term.

“This is a guy who said I have no knowledge of foreign affairs,” Senator Barack Obama says, his voice hitting a high C on the incredulity scale, before he adds: “Well, John McCain was arguing for a war that had nothing to do with 9/11. He was wrong, and he was wrong on the most important subject that confronted our nation.”

The crowd rises, clapping and cheering at this pleasing whiff of partisan buckshot.

As Mr. Obama stands poised to claim the crown of presumptive Democratic nominee, he is, gingerly, fitting himself with the cloth of a partisan Democrat despite having long proclaimed himself above such politics. That his shift in tone was inevitable and necessary, particularly as Mr. McCain, the presumptive Republican nominee, slashes at Mr. Obama as weak on Iran and terrorism, does not entirely diminish the cognitive dissonance.

For 17 months, Mr. Obama, of Illinois, has changed remarkably little about his stump style. He projects the image of a post-partisan candidate with the confidence of a man convinced he holds a copyright.