With his campaign for the Democratic presidential nomination lagging in the polls, Senator Obama of Illinois is sharpening his attacks on Senator Clinton’s stance on the Iraq War and adding a new charge that she has been dishonest when explaining a key Senate vote on Iraq.

Speaking at DePaul University in Chicago yesterday, Mr. Obama did not mention the front-runner in the Democratic presidential contest, Mrs. Clinton, by name. However, his criticism of her was unmistakable as he honed in on legislators who backed a 2002 congressional resolution authorizing the use of force in Iraq.

“Without that vote, there would be no war. Some now seek to rewrite history. They argue they weren’t really voting for war. They were voting for inspectors, or they were voting for diplomacy. But the Congress, the administration, the media, and the American people all understood what we were debating in the fall of 2002,” Mr. Obama declared. “This was a vote about whether or not to go to war. That’s the truth as we all understood it then and as we need to understand it now. We need to ask those who voted for the war, ‘How can you give the president a blank check and then act surprised when he cashes it?'”

“The American people weren’t just failed by a president. They were failed by much of Washington,” Mr. Obama said.

As the war has dragged on, Mrs. Clinton has insisted that her vote in October 2002 for a measure officially titled, “A joint resolution to authorize the use of United States Armed Forces against Iraq,” was intended to give President Bush added negotiating power with Saddam Hussein and to get U.N. weapons inspectors back into Iraq. She has said that, to her surprise, Mr. Bush abused that authority.

A spokesman for Mrs. Clinton, Phil Singer, said voters are more interested in how to bring American troops home from Iraq than in who said what about the issue five years ago.

“This is more of the same from Senator Obama,” Mr. Singer said. “We believe voters are focused on the future and on ending the war in Iraq. Increasingly, Americans think Senator Clinton is the candidate with the strength and experience to do so.”

Mr. Obama’s speech yesterday came on the fifth anniversary of his appearance at an anti-war rally where he forcefully denounced the planned invasion of Iraq and predicted, accurately, that sectarian strife would result. The Illinois senator, whose presidential campaign has been stymied by concerns over his lack of experience, insisted that he exhibited greater foresight than those with longer résumés.

“There is a choice that has emerged in this campaign, one that the American people need to understand. They should ask themselves who got the single most important foreign policy decision since the Cold War right, and who got it wrong,” he said.

In his speech yesterday, Mr. Obama also urged the abolition of all nuclear weapons. “It’s time to stop giving countries like Iran and North Korea an excuse. It’s time for America to lead,” he said.

Mr. Obama’s call for America to “firmly abide” by the Non-Proliferation Treaty’s goal of ridding the world of nuclear weapons stirred some nervousness in Israel, where a nuclear deterrent is seen as crucial to avoiding invasion by the large armies in neighboring Arab states.

“What’s disturbing is Obama, like [President] Carter 30 years ago, is reflecting that very naïve approach that takes the rhetoric of Third World dictators at face value,” a professor who studies conflict management at Bar Ilan University in Israel, Gerald Steinberg, said. He said it is a “myth” that rogue states seek nuclear weapons because America or Israel has them. “The Iranians, the Syrians, the North Koreans would probably pursue these weapons more and be even more motivated if America didn’t have them,” the professor said.

An aide to Mr. Obama said there was no need for supporters of Israel to be jittery. “This is a net positive for Israel,” the adviser, who asked not to be named, said. “Barack takes very seriously the security of the United States and of Israel.”

The adviser to Mr. Obama said his call for sanctions was limited to states that have signed the anti-proliferation pact. Israel, which has never signed, would benefit from aspects of Mr. Obama’s plan aimed at securing “loose nukes” in the former Soviet Union, the adviser said. “That material is being actively sought by terrorists, and the place they want to use that is Israel and the United States,” he added.