Mr. Obama’s hands have not always been clean in this regard. He was called out earlier for saying, incorrectly, that Mr. McCain supported a “hundred-year war” in Iraq after Mr. McCain said in January that he would be fine with a hypothetical 100-year American presence in Iraq, as long as Americans were not being injured or killed there.

More recently, Mr. Obama has been criticized for advertisements that have distorted Mr. McCain’s record on schools financing and incorrectly accused him of not supporting loan guarantees for the auto industry  a hot topic in Michigan. He has also taken Mr. McCain’s repeated comments that American economy is “fundamentally sound” out of context, leaving out the fact that Mr. McCain almost always adds at the same time that he understands that times are tough and “people are hurting.”

But sensing an opening in the mounting criticism of Mr. McCain, the Obama campaign released a withering statement after Mr. McCain’s appearance on “The View.”

“In running the sleaziest campaign since South Carolina in 2000 and standing by completely debunked lies on national television, it’s clear that John McCain would rather lose his integrity than lose an election,” Hari Sevugan, a spokesman for the Obama campaign, said in a statement.

At an event in Dover, N.H., a voter asked Mr. Obama when he would start “fighting back.” Mr. Obama, who began his own confrontational advertising campaign Friday, said, “Our ads have been pretty tough, but I just have a different philosophy that I’m going to respond with the truth.”

“I’m not going to start making up lies about John McCain,” Mr. Obama said.

The McCain advertisements are devised to draw the interest of bloggers and cable news producers  but not necessarily always intended for wide, actual use on television stations  to shift the terms of the debate by questioning Mr. Obama’s character and qualifications.

Mr. Sipple, the Republican strategist, voiced concern that Mr. McCain’s approach could backfire. “Any campaign that is taking liberty with the truth and does it in a serial manner will end up paying for it in the end,” he said. “But it’s very unbecoming to a political figure like John McCain whose flag was planted long ago in ground that was about ‘straight talk’ and integrity.”