Barack Obama voted Wednesday afternoon for a surveillance bill that includes a provision he once opposed, giving Republicans ammunition in their argument that he is shifting positions to appeal to political moderates.

The bill, which passed the Senate and is expected to be signed by President Bush, would set new rules for government eavesdropping, and includes a measure giving immunity to telecommunications companies that helped the government eavesdrop on Americans without court permission after Sept. 11.

Obama voted for an amendment earlier Wednesday that would have stripped the bill of such immunity -- but after it failed to pass, he still supported the overall bill.

The presumptive Democratic presidential nominee, a senator from Illinois, angered liberals when he first voiced support for the compromise last month.

He said at the time that it was a "close call," but that the legislation made sure that the president "can't make up rationales" for wiretapping without warrants.

He said the legislation "met my basic concerns."

Obama fought against such immunity last year. He released a statement in December saying he "unequivocally opposes giving retroactive immunity to telecommunications companies. ... Granting such immunity undermines the constitutional protections Americans trust the Congress to protect. Senator Obama supports a filibuster of this bill, and strongly urges others to do the same."

He reaffirmed his opposition in a February statement that said, "There is no reason why telephone companies should be given blanket immunity to cover violations of the rights of the American people."

Liberal bloggers swarmed Obama's own campaign Web site to criticize him for the surveillance vote.

Before Obama even arrived at Capitol Hill on Tuesday, McCain's campaign needled him for supporting the compromise.

"A few short months ago, Barack Obama outwardly opposed terrorist surveillance legislation, saying that he would filibuster any bill that includes immunity for American telecommunications companies," spokesman Tucker Bounds said. "Today, the U.S. Senate will approve legislation providing the immunity Barack Obama supposedly opposed, and despite his promise, he will not support a filibuster.

"What Barack Obama will do is show that he's willing to change positions, break campaign commitments and undermine his own words in his quest for higher office."

McCain later told reporters in South Park, Pa., that he and Obama "are still in strong disagreement on the issue of immunity for the telecommunications" companies, since Obama still technically opposes the immunity provision and he supports it.

But he said that on the Foreign Intelligence Surveillance Act overall, Obama did flip. He said that's "not the first change in position."

McCain missed the vote on the surveillance bill, as he campaigned in Ohio and Pennsylvania.

Obama has caught flak for several positions he's taken in recent weeks.

He supported a Supreme Court decision that invalidated the Washington, D.C., handgun, even though his campaign last year said that ban was "constitutional."

He said last week he might "refine" his Iraq policies after traveling to the Middle East this summer, though he later insisted he was not talking about changing his stated plan to remove all U.S. brigades from Iraq within 16 months of taking office.

The McCain campaign on Tuesday released a list of 17 alleged flip-flops.

But Obama battled the accusations at a town hall meeting Tuesday in Georgia.

"This whole notion that I am, you know, shifting to the center or that I am flip-flopping or this and that and the other. The people who say this apparently haven't been listening to me," he said.

Obama said he is "no doubt progressive," and that "the notion that somehow that's me trying to look like I'm more centered -- more centrist -- is just not true."

FOX News' Trish Turner and Bonney Kapp and The Associated Press contributed to this report.