But there are potential risks.
"The peril is not as much among the progressive base as it is among the general electorate," says David Sirota, author of the book "The Uprising." "What Obama is saying is, 'I'm a vacillating politician.' The public does not like politicians who try to nuance their way out of principled positions."
Obama has shown some responsiveness to the left's push back. In one recent brouhaha, retired Gen. Wesley Clark, an Obama supporter, took after McCain on a Sunday talk show, saying that the senator's Vietnam experience did not necessarily qualify him to be commander in chief. An Obama spokesman disavowed General Clark's remark, but after liberal standard-bearer MoveOn.org defended Clark, Obama backed off the condemnation of the general.
"That was a significant moment when Obama realized he had gone too far – or at least [he] pulled back," says Matt Stoller, a liberal blogger and political consultant. "If he continued to betray the core values of some of his most ardent supporters, I think it would eventually become a problem."
Arianna Huffington, doyenne of the liberal blogosphere, is less charitable. "The Obama campaign is making a very serious mistake," she writes. "Tacking to the center is a losing strategy."