The Bloomberg campaign got some bad news over the weekend when a former Republican leader of the City Council, Thomas Ognibene, made it official: He plans to challenge Mayor Bloomberg in the primary.

“The real, dramatic announcement will be in a couple of weeks, but everybody knows this is it: I’m running,” the 61-year-old former councilman said in an interview with The New York Sun. “I am running because the Republican Party deserves a candidate for mayor. I had talked about running last year, had withdrawn for personal reasons from an active campaign. But now I am back and I am going to hold the mayor’s feet to the fire.”

While political analysts – and even Mr. Ognibene – don’t think he will be elected mayor in a town where registered Democrats outnumber Republicans by five to one, the councilman is considered a formidable opponent who could end up as a spoiler. If the city’s half-million registered Republicans decided to avenge Mr. Bloomberg’s property tax hike or his smoking ban, Mr. Ognibene could win the primary.

Perhaps a more serious threat to Mr. Bloomberg is the possibility that Mr. Ognibene could lose the GOP primary but then run on the Conservative Party line. That would allow him to siphon the very outer-borough and Republican votes Mr. Bloomberg is counting on to help him win the general election.

Mr. Ognibene wouldn’t say whether that was his intention. What he did say was that he has been in discussions with the head of the Conservative Party, Michael Long, and expects to get the party’s endorsement.

“My theory is that Tom Ognibene has the Conservative line already locked up,” a Baruch College political-science professor, Douglas Muzzio, said. “To the base of Republican voters in the city, a vote for Tom makes more sense than a Bloomberg vote does. We could see Tom run in the general election as Conservative Party candidate, take 5% of the vote by wooing Republicans and people who are disillusioned by Bloomberg, and then a Democrat ends up winning.”

In the 2001 general election for mayor, out of 1.5 million votes cast, the Conservative Party’s candidate, Terrance Gray, won fewer than 4,000. Mr. Gray, however, was not as well known as Mr. Ognibene.

And if it seems far-fetched that Mr. Ognibene, who said he has not yet started raising money for his campaign, could topple the billionaire mayor, consider the 1969 election, when Mayor Lindsay, a liberal Republican, was upset in the Republican primary by a state senator from Staten Island, John Marchi. Lindsay ended up having to run as an independent with the backing of the Liberal Party. He did win re-election, but with barely 40% of the vote – and largely because the Democratic candidate, Mario Procaccino, and Mr. Marchi split the conservative vote.

“Do I think the likely outcome will still be Fernando Ferrer and Bloomberg in the general election? Yes,” Mr. Muzzio said. “But Ognibene is potentially a knife in the heart for Bloomberg. He represents a significant threat.”

Mr. Ognibene expects to qualify for the 6-to-1 match in public campaign subsidies for which candidates, who are up against big-money rivals like Mr. Bloomberg, can qualify. He said he expects to raise enough money to hit the $250,000 threshold so he would have more than $1 million to compete with the mayor.

Political analysts on both sides of the aisle see Mr. Ognibene as a smart, savvy, articulate politician. He rose through the ranks of the state’s Conservative Party to become a Republican insider who worked to elect Governor Pataki and become a member of Mayor Giuliani’s inner circle. He represented a working-class area of Queens, known as the “Archie Bunker district,” for a decade in the City Council, and he was able to convince many Democrats in his district to keep voting for him.

That is not to say that Mr. Ognibene has not been without his political missteps. At a press conference after the fatal shooting of Amadou Diallo by four white police officers, for example, Mr. Ognibene said that members of minority groups, instead of blasting Mr. Giuliani’s tough crime policies, ought to thank the mayor for saving their lives by making communities safer. Many people, at the time, saw his comments as misplaced and possibly racist.

“You had a terrible shooting, everyone agreed it was a terrible issue,” Mr. Ognibene told the Sun yesterday. “Everyone wanted to lay this at the feet of Giuliani. If you look statistically, the policies and practices of Giuliani provided more safety on the streets in minority communities than any previous mayor. Whether it would have been more effective to say so two months later, I don’t know. At the time I thought it was the right thing to say, and I said so with good intentions.”

When term limits forced him to leave the council, Mr. Ognibene expected Mr. Pataki to appoint him to the New York State Court of Claims. Then his name surfaced in a bribery investigation involving the city’s Buildings Department. Mr. Ognibene was never charged with a crime, but his chances of an appointment to the court seemed to evaporate. Since that time, Mr. Ognibene has periodically floated the idea of running against Mr. Bloomberg for the Republican nomination.

“I think that Tom Ognibene has a very important message, and I think he’s voicing a lot of the frustrations that rank-and-file Republicans have had with the administration over the last three years,” the current Republican minority leader of the City Council, James Oddo, said. “His presence in the race creates all kinds of problems for the administration.”

The Bloomberg campaign, for its part, wants to stick to the facts: On this mayor’s watch, crime in the city has been at historic lows, the economy is back on track, and Mr. Bloomberg has gained control of the schools. “If Ognibene can top the mayor’s record of reducing crime, getting the city’s economy back on track, and reforming the schools, he is welcome to try,” the mayor’s spokesman, Edward Skyler, told the Sun.

Mr. Ognibene sees the argument as a more fundamental one. “My concern is that people understand the Republican Party is the party that saved the city,” he said. “It isn’t always about winning. The only vote you waste is when you vote for someone you don’t identify with. Unless someone is there to hold Bloomberg’s feet to the fire, he is going to keep distancing himself from our party. He has to be proud to be a Republican.”