The first big legal battle of the 2008 campaign is breaking out in Michigan, where Democrats are asking a federal court to block an alleged Republican plan to disenfranchise voters who lost homes to foreclosure.

Republicans, who vehemently deny the existence of any such effort, are vowing to file a libel suit against an online publication, the Michigan Messenger, which quoted a county GOP chairman as saying the party planned to study foreclosure lists to challenge voters at the polls.

“There’s no truth to the story. It’s a complete fabrication,” the state Republican chairman, Saul Anuzis, told reporters yesterday. “There has never been a plan to use foreclosure lists to challenge voters.”

An article published last week on the Messenger’s site said the Republican chairman in Macomb County, Mich., James Carabelli, disclosed the foreclosure-related plan in a telephone interview. “We will have a list of foreclosed homes and will make sure people aren’t voting from those addresses,” he was quoted as saying.

Mr. Carabelli has since denied making the statement. He did not respond to phone or e-mail messages seeking comment for this article.

Democrats said they were aware of the denials but simply did not believe them in light of Republican Party’s history of challenging voter qualifications at the polls.

“We haven’t fallen off the back of the turnip truck, here,” a lawyer for Senator Obama’s presidential campaign, Robert Bauer, said. “This is a standard operating procedure of the Republican Party. … We’re not going to accept that because they were embarrassed publicly they are going to reform.”

In a conference call with reporters, Mr. Bauer was pressed to identify what evidence, beyond the disputed quote from Mr. Carabelli, supported the Democratic lawsuit filed yesterday. The Obama campaign lawyer pointed to a quote in which a Republican attorney in Michigan said he expected the party to send mailings to voters and to challenge voters in instances where the letters were undeliverable. Mr. Bauer also noted a report in an Ohio newspaper in July that a local GOP official declined to rule out challenges tied to foreclosures.

Mr. Bauer said the alleged plan to base voter challenges on foreclosure lists smacked of mailing-based practice known as caging. The Republican National Committee agreed to limit such activities as part of a federal court consent decree in 1982 after a disputed election in New Jersey in which letters were allegedly targeted to African-American neighborhoods. The Democratic attorney called the alleged Michigan version of caging “particularly ugly,” “especially repellant,” and “horrific.”

Under Michigan law, a voter who moves his residence within the state in the 60 days before an election can still vote at his or her old polling place.

While Mr. Anuzis initially said he hoped the libel suit against the Messenger would be filed yesterday, another Republican official said a demand for a retraction would have to be sent to the publication first. Michigan law requires that a potential libel plaintiff make such a demand and wait a reasonable amount of time before going to court.

However, the president of the nonprofit organization that owns the Messenger said yesterday that no retraction will be forthcoming. “We stand by our story,” David Bennahum, the president and CEO of the Center for Independent Media, told The New York Sun. “We’re not issuing a retraction.”

Republican officials have sought to discredit the Messenger as a biased outlet seeking to score political points for its financial backers, including a Manhattan financier and philanthropist, George Soros.

“We basically have a liberal blogger who’s been funded by some left-wing billionaires who’ve been pushing a liberal agenda,” Mr. Anuzis said. “I think there may have been some leading questions. They were trying to get him to mention the word, ‘foreclosure.’ … I think this was pretty much a set-up.”

Mr. Bennahum confirmed that his center is supported by a charity founded by Mr. Soros, the Open Society Institute, but said that doesn’t mean the group is directed by the liberal financier. “The Open Society Institute is one of our 60 funders,” Mr. Bennahum said. “They provide less than 5% of our funding.”