In the war over ballot-access petitions, Mayor Bloomberg finds himself fighting on two fronts. Already embroiled in a battle over the Republican Party primary with a Queens lawyer, Thomas Ognibene, Mr. Bloomberg also faces a challenge of the petition signatures gathered for him on the Independence Party line.

A former Manhattan vice chairman of the Independence Party, Michael Zumbluskas, said he filed in state Supreme Court in Manhattan yesterday a challenge to the 4,839 signatures collected by that party to put Mr. Bloomberg on its ballot line in November.

To appear on the ballot in a citywide race as a Democrat or Republican, a candidate must collect 7,500 petition signatures from the party’s registered voters. For smaller parties, the threshold is 5% of the registered membership. According to a spokeswoman for the Independence Party, Mr. Bloomberg needed around 3,900 signatures to win the party’s nomination for the general election. The mayor, the spokeswoman said, has “almost 900 over what has been required.”

Mr. Zumbluskas, however, is questioning the authenticity of hundreds of those petition signatures. Comparing 3,000 of the signatures on the Bloomberg petitions to signatures on file with the Board of Elections – which maintains scanned copies of New Yorkers’ voter-registration forms – Mr. Zumbluskas said he found “at least four or five hundred” that did not match.

The chairwoman of the New York County Independence Party, Cathy Stewart, said: “The petitions are clean as a whistle.” She added: “Mr. Zumbluskas should knock himself out.”

A spokesman for the Bloomberg campaign, Stuart Loeser, said: “People are free to file objections, and those signatures should be held up to the standards of election law.”

Mr. Zumbluskas, a political consultant who said he is not working for any mayoral candidate, portrays the challenge of Mr. Bloomberg’s Independence Party petitions as a way both to keep the mayor off the Independence line and to lay the foundation for a federal lawsuit that would force the city to change its election laws, to make it easier for candidates to get on the ballot.

Mr. Zumbluskas – who described himself as “the head of the opposition” in New York City to the Independence Party leader Lenora Fulani – said he was also hoping to effect reform of the party, which he described as “a cult.”

Ms. Fulani has long been criticized for, among other things, making remarks widely considered to be anti-Semitic. Mr. Bloomberg has denounced those remarks but said they are not representative of the party – to which he has donated hundreds of thousands of dollars, and which, in the tight 2001 mayoral election, provided him with more than 59,000 votes.

Ms. Stewart disputed Mr. Zumbluskas’s assessments, saying: “The Independence Party is not a cult. It’s the third largest political party in the state of New York.” She dismissed Mr. Zumbluskas as “part of the nag network of dissidents inside the Independence Party.”

Meanwhile, as Mr. Zumbluskas raised questions about the authenticity of the mayor’s Independence Party petitions, Mr. Bloomberg was accusing the Ognibene campaign of fraud in the attempt to force a GOP primary. Earlier this week, Mr. Bloomberg said he would challenge the nearly 8,100 signatures filed by Mr. Ognibene, in a move that threatened to keep him off the ballot.

“There’s 500,000 registered Republicans in the city,” Mr. Bloomberg said. “It should be relatively easy, if you have any support, to get 7,500. This guy couldn’t get 7,500 because people don’t want to see him on the ballot.

“And you know you have to have some standards … and it shouldn’t be done with fraud,” the mayor continued. “When you have one page where one person’s obviously written a bunch of names of people who aren’t even in town, that’s not what we should have.”

Responding last night, Mr. Ognibene said: “He’s obviously in a panic because people are beginning to understand that he’s afraid of a Republican primary.”

The challenger, who expects to have the Conservative Party line on the November ballot, also said: “As far as getting signatures goes, if there are 500,000 Republicans, how come the mayor is spending $30 million and with the support of four and a half counties got only 24,000?”