Senator Clinton’s campaign is mounting a concerted and multi-pronged challenge to Senator Obama’s claims that he has been a steadfast opponent of the Iraq War. “When he became a senator, he didn’t go to the floor of the Senate to condemn the war in Iraq for 18 months. He didn’t introduce legislation against the war in Iraq. He voted against timelines and deadlines initially,” Mrs. Clinton said yesterday on NBC’s “Meet The Press.”

“You have two different story lines here. You have Senator Obama’s story line, the speech he gave in ’02, to his credit, which then was not followed up on. By ’03, it was off his Web site. By ’04, he was saying he didn’t know how he would vote and that he basically agreed with George Bush on the conduct of the war. There were others … who voted against it, spoke out against it, and never wavered over that period of time,” Mrs. Clinton said. She acknowledged that she also failed to oppose war funding, but added, “I’m not giving you a story line that doesn’t hold up.”

Mrs. Clinton questioned Mr. Obama’s assertion that his apparent waffling on the war in a couple of interviews in 2004 was an effort to avoid embarrassing the Democratic Party’s presidential and vice presidential nominees. “He’s always said he doesn’t take positions for political reasons. That is a political explanation,” the New York senator said.

On the Web and in e-mails to journalists, Mrs. Clinton’s campaign pressed the attack further by noting that Mr. Obama voted for $300 billion in war funding and that he voted to confirm General George Casey as Army chief of staff, while Mrs. Clinton opposed the nomination. The Clinton campaign also organized a conference call where her supporters continued the assault on Mr. Obama.

“There’s nothing unique about his Iraq position except a speech he made that he never followed up on,” a former State Department spokesman, James Rubin, told reporters. The telephone session was unusual in that it took place even as Mrs. Clinton was taking questions from voters at a public forum in Columbia, S.C.

Mr. Rubin also said the exchange on Iraq at a recent New Hampshire debate showed Mrs. Clinton’s plan to withdraw troops from Iraq was “different” from Mr. Obama’s because she stated America needed to be cautious in pulling out. The former state department official neglected to note that the Illinois senator said at that debate that “we should be as careful getting out as we were careless getting in.”

At a stop in Las Vegas yesterday, Mr. Obama blasted his rival for the fusillade of attacks. “Senator Clinton started off trying to make history and now she’s trying to rewrite it. She’s trying to rewrite it about my record and hers,” the Illinois senator said. He said Mrs. Clinton was taking “a half a sentence” from 2004 out of context in what he called a “ludicrous” attempt to assert that he and she were in agreement.

“They have decided to run a relentlessly negative campaign,” Mr. Obama said. “I gather that she’s determined that instead of trying to sell herself on why she would be the best president, she’s trying to convince folks that I wouldn’t be a good one.”