ASHINGTON, Nov. 17 — Howard Dean, the former governor of Vermont, started a tough advertising campaign against Representative Richard A. Gephardt in Iowa on Monday that highlights Mr. Gephardt's role in drafting a resolution on a war in Iraq.

The commercial goes to the heart of what Mr. Gephardt's own advisers say is his biggest vulnerability in a state he won in his 1988 presidential bid: the role he played in delivering the resolution for the White House.

The advertisement is the most direct challenge a Democrat has made in the presidential primary season and underscores the degree the Iowa race has become a two-man fight.

Dr. Dean's commercial shows a photograph of Mr. Gephardt, who is from Missouri, standing with President Bush on Oct. 2, 2002, when the president and Congressional leaders announced that the war resolution was at hand.

An announcer says, "October 2002. Dick Gephardt agrees to co-author the Iraq war resolution, giving George Bush the authority to go to war. A week later, with Gephardt's support, it passes Congress."

After the announcer reminds viewers that Mr. Gephardt voted for the president's $87 billion financial package for the occupation and rebuilding of Iraq and Afghanistan, the commercial switches to Dr. Dean.

"I opposed the war in Iraq. And I'm against spending another $87 billion there," he says.

Dr. Dean's campaign sent a brochure to Iowa voters striking a similar theme last week in a clear effort to stoke antiwar sentiment against Mr. Gephardt. The strategy appeared to diverge from Dr. Dean's recent effort to position himself as the inevitable nominee by focusing his attention more on President Bush than on his primary opponents.

The latest poll in Iowa shows Mr. Gephardt leading Dr. Dean, who on Monday night earned the endorsements of Elijah E. Cummings, chairman of the Congressional Black Caucus, and David Wu, chairman of the Congressional Asian Pacific Caucus.

Steve Murphy, Mr. Gephardt's campaign manager, indicated that he would not respond in kind. "One thing we're not going to do," he said, "is allow Howard Dean to turn the Iowa Democratic caucus into a negative, tit-for-tat campaign."

Officials with Dr. Dean's campaign said the advertisement should not be called negative. "It's an honest disagreement on an important matter of policy," said Steve McMahon, Dr. Dean's media consultant.

Analysts said the strategy was somewhat of a gamble because voters in Iowa are believed to be averse to political advertising they perceive as negative.

"I would say it is a risky move that has upside if it's successful and drives Gephardt down without hurting Dean," said Douglas E. Schoen, a pollster who is not representing any of the candidates. "The downside of it is it could weaken both, or potentially just Dean."

The Dean campaign spent roughly $250,000 on the commercial, officials said, and will run it for 7 to 10 days.

Meanwhile, officials with the campaign of Senator John Kerry of Massachusetts announced on Monday an Iowa commercial taking on President Bush on environmental issues.

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