"When he was running for his party's nomination, he walked away from that commitment. He said he wouldn't even support his own legislation if it came up for a vote," Obama said. "If we are going to solve the challenges we face, we can't vacillate, we can't shift depending on our politics."

An Obama spokesman said the Illinois senator was referring to a statement McCain made during the Jan. 30 Republican debate at the Reagan presidential library in which McCain was asked whether he would vote for comprehensive immigration legislation he had sponsored.

"No, I would not, because we know what the situation is today," McCain responded at the debate. "The people want the border secured first."

The accusation rapidly stirred an e-mail skirmish between the two campaigns.

McCain's camp sent out a news release charging that Obama had undermined attempts to pass immigration reform legislation in 2006 by supporting amendments opposed by business interests that the McCain campaign called "poison pills."

The Obama camp responded with a statement McCain made on the Senate floor just before the final vote on the legislation in which the Arizona senator commended Obama for "working to ensure this bill moved successfully intact through the legislative process."