Julie Bosman reported from Kaukauna, and John M. Broder from Washington. KAUKAUNA, Wis. – Confronted with one of the trickiest problems in...

Julie Bosman reported from Kaukauna, and John M. Broder from Washington.

KAUKAUNA, Wis. – Confronted with one of the trickiest problems in politics – when to ignore rumors and misrepresentations and when to risk giving them greater visibility by rebutting them – Sen. Barack Obama’s presidential campaign began to push back aggressively on Thursday against what it said were unfounded and potentially damaging reports.

Obama’s campaign unveiled a new Web site on which it listed five sets of rumors about Obama and his wife, Michelle, along with responses intended to establish that they are baseless and false.

The Obama campaign encouraged supporters to read each rumor and the corresponding facts debunking it, and then to e-mail the entries to their entire address books.

By Thursday evening, more than 20,000 people had registered at the site, and more than 18,000 e-mail messages were sent, said a spokesman, Tommy Vietor.

"The Obama campaign isn’t going to let dishonest smears spread across the Internet unanswered," Vietor said in a statement. "Whenever challenged with these lies, we will aggressively push back with the truth and help our supporters debunk the false rumors floating around the Internet."

Obama has been dogged by potent, fast-moving rumors about his religion, his birthplace and his patriotism, to name a few, for more than a year. More recently his campaign has confronted persistent but unsubstantiated reports about Michelle Obama using angry and derogatory language about white people.

Until now, the Obama campaign has tried to confront Internet rumors with a more local, less visible approach, quietly encouraging supporters to forward misleading e-mail messages to their local field organizers, who in turn would notify the campaign’s research department. Then the research department would respond with a factual rebuttal, which would then be e-mailed back to every recipient of the original e-mail message.

While the campaign considered this tactic effective in the early primaries, especially in the days leading up to the Iowa caucuses in January, the general election was seen as requiring a more robust response mechanism, so the campaign created the Fight The Smears Web site. Details of fightthesmears.com were first reported by Time magazine.

The campaign unveiled the new Web site amid indications the general election could be as much of a slash and burn affair as any of its recent predecessors, despite assertions from both Obama and his Republican rival, Sen. John McCain, that they intend to elevate the political discourse.

On Wednesday, James A. Johnson, a longtime fixture in Democratic political and business circles, stepped down as the head of Obama’s vice-presidential search committee after Republicans used news reports about Johnson’s business dealings to suggest that he had ethical problems.

On Thursday, McCain, at a news conference, raised questions about another member of the search committee, Eric Holder, noting that he had played a role in President Bill Clinton’s last-minute pardon of the fugitive financier Marc Rich in 2001.

Obama’s campaign and its allies have displayed an aggressive streak this week as well, hitting McCain hard on Wednesday after he replied to a question about when U.S. troops might come home from Iraq by saying it was "not too important," then suggesting that what was important was the level of casualties.

In a conference call about the statement, Obama’s advisers on several occasions referred to McCain as being "confused" about some key Middle East questions, a statement some took to be a veiled reference to his age and acuity.

But the final straw for Obama, his aides said, was the story circulating on conservative blogs that a video existed showing Michelle Obama making a racially tinged speech at their former church, Trinity United Church of Christ in Chicago.

Obama was visibly irritated by the rumor when a reporter asked him about it last week.

"It is a destructive aspect of our politics right now," Obama told reporters last week as he flew through Virginia. "And simply because something appears in an e-mail, that should lend it no more credence than if you heard it on the corner. And you know, presumably the job of the press is to not go around and spread scurrilous rumors like this until there’s actually anything, one iota of substance or evidence that would substantiate it."

The Web site devotes its most prominent space on the home page to that rumor. "The Smear," it begins. After describing the smear in a point-by-point fashion it concludes, "The Truth: No Such Tape Exists."

© The New York Times. All rights reserved. This article originally appeared in The New York Times.