WASHINGTON — Senator Obama may have joined Senator Clinton atop the polls in Iowa, but to John Edwards, it seems only one candidate is standing between him and the first crucial prize in the Democratic primary fight: the former first lady.

Strategically, the Mr. Edwards has focused almost exclusively on drawing contrasts with Mrs. Clinton, virtually ignoring Mr. Obama’s rise in recent surveys.

That tactic was particularly in evidence yesterday when the Edwards campaign launched its latest campaign Web site, americabelongstous2008.com, which urges Democrats to pledge not to vote for any candidate who accepts contributions from political action committees or Washington lobbyists. Left unsaid is that of the top three Democrats, Mrs. Clinton is the only candidate who would be affected; like Mr. Edwards, Mr. Obama has already sworn off such donations after accepting them in previous campaigns.

Although the site does not mention Mrs. Clinton by name, the former North Carolina senator has long criticized her for defending a “corrupt” system in Washington that he says has been an impediment to meaningful change, such as achieving universal health care. Earlier this month, the Edwards campaign launched another Web site aimed at Mrs. Clinton, plantsforhillary.com, which poked fun at disclosures that her staff had “planted” questions at campaign events. The site was taken down within days.

Advisers to Mr. Edwards downplay his emphasis on Mrs. Clinton, saying he has also taken on Mr. Obama’s message of change by advocating confrontation over compromise with the entrenched special interests that both men say are blocking large-scale reforms.

“There is a distinct and stark difference between what he is talking and what we’re talking about,” a senior adviser to Mr. Edwards, Joseph Trippi, said.

Mr. Edwards has also criticized Mr. Obama’s health care plan for lacking a mandate for people to buy insurance, but Mrs. Clinton, whose proposal is similar to Mr. Edwards’s, has been making the same argument with more force against the Illinois senator in recent days.

Mr. Edwards has acknowledged that he is closer to Mr. Obama than he is to Mrs. Clinton on the issues, telling reporters in Iowa earlier this month, “The differences between Senator Clinton and myself are much more dramatic than the difference between Senator Obama and myself.”

Yet with Mr. Obama tied or leading in the latest polls there, focusing solely on Mrs. Clinton may not be enough for Mr. Edwards, who surveys indicate is a few points behind both his rivals. “At some point soon, he’s going to have to find some angle to distinguish himself from Senator Obama,” a professor of political science at the University of Iowa, Peverill Squire, said.

Mr. Edwards has been building an organization in the first voting state virtually since the 2004 presidential campaign ended, and his campaign has made no secret of the fact that success in the state is imperative.

Mrs. Clinton’s campaign declined to comment on the new Edwards Web site. While she has responded to his criticism, she has not gone out of her way to attack him.