Mayor Bloomberg won his fourth Republican Party county endorsement Tuesday night, ending a five-week whirlwind that started when the Republican organization at Queens endorsed one of his challengers, Thomas Ognibene.

The endorsement from the Staten Island GOP removes much of the doubt Mr. Bloomberg was facing just weeks ago about whether he would get full support from the establishment of his own party. Since then, he has received backing from GOP organizations in Brooklyn, Manhattan, and the Bronx. The latest backing also gives the mayor more foot soldiers in a borough that is crucial to winning November’s mayoral election.

Mr. Bloomberg, The New York Sun has learned, has also won over Council Member Andrew Lanza, one of three Republicans in the 51-member body.

Mr. Lanza’s endorsement comes after a rocky relationship with the mayor, dating back to three years ago when Mr. Lanza and the Republican minority leader in the council, James Oddo, stormed out of Gracie Mansion after the mayor praised Democrats for passing his 18.5% property tax increase.

During an interview yesterday, Mr. Lanza, who won his last election on Staten Island’s South Shore with 82% of the vote, said he decided to endorse Mr. Bloomberg partly because the mayor had cracked down on overdevelopment in his district, where builders are squeezing in townhouses and new homes on every matchbook-sized plot of land.

Mr. Lanza, who in the past has criticized the mayor on a range of issues, rode with the Mr. Bloomberg from a civic association event at Staten Island’s South Shore to the Republican Party’s office to introduce him to the executive committee. Political observers said that the new support allows Mr. Bloomberg to exhale and focus on other matters.

“The first thing you do is lock up and secure your base and the Republican county organizations,” a Baruch College political science professor, Douglas Muzzio, said yesterday. “The borough parties are his organizational base. He needs them to get out the troops.”

The county endorsements were fast tracked this year because of what happened in Queens. In most election seasons, county organizations announce their endorsements in April or May just in time to print up campaign brochures. In Manhattan, for example, the county party prints out voter guides listing the names of the candidates they’ve endorsed.

Yesterday, Mr. Ognibene and another Republican, an investment banker named Steve Shaw, who is also planning to run in a primary against the mayor, said they were still moving forward with their campaigns. Both men dismissed the notion that they would not collect the 7,500 signatures needed to get their names on the ballot for the September primary.

The pair have raised a combined $54,000, compared to the millions of dollars the mayor plans to spend. And, in a recent Quinnipiac University poll Mr. Bloomberg beat out Mr. Ognibene, a former minority leader in the council, by a 65% to 16% margin.