Mayor Bloomberg’s opponent in the Republican primary of 2001, Herman Badillo, endorsed the mayor yesterday and, in an interview with The New York Sun, pledged to campaign with Mr. Bloomberg in battleground neighborhoods he needs for re-election.

A year ago, Mr. Badillo marched with the mayor in the Three Kings Parade in East Harlem, and the two men discussed the scourge of “social promotion” in the city’s schools. At the time, though Mr. Badillo embraced Mr. Bloomberg’s efforts to hold children back in third grade if they couldn’t do the work, he quickly added: “But I am not endorsing him.”

Apparently, the mayor’s progress in the schools has brought Mr. Badillo around. “I have been working with Mayor Bloomberg and Chancellor Klein on social promotion, and I think there is progress,” he told the Sun yesterday. “Take a look at the percentage of kids graduating from high school. It is up 4%.”

The endorsement from a fellow Republican – albeit, like Mr. Bloomberg, a convert to the GOP – comes as the mayor is fighting an opponent on his right flank. The former Republican leader of the City Council, Thomas Ognibene, has said he will challenge the mayor in a Re publican primary and, as the Sun reported yesterday, has already secured the support of the Queens County Republican Party. Mr. Badillo’s nod could take some of the sting out of that.

It also beefs up Mr. Bloomberg’s education reform credentials, political analysts said. Mr. Badillo is seen as one of the city’s leaders in education.

“The issue of whether Mike Bloomberg is a success in education is up to interpretation. The numbers cut both ways,” the president of the Advance Group, Scott Levenson, a political consultant, said. “What matters here, not the facts of the case but the perception of how he has done. Badillo’s support goes a long way toward shaping public opinion in Bloomberg’s direction.”

Mr. Badillo has been a fixture in New York politics. He was the Bronx borough president in the 1960s, and the first Puerto Rican elected to Congress. He ran for mayor several times, ran for comptroller, and served as chairman of the board of the City University of New York. Because of his work at CUNY and the subsequent tightening of standards there, he is seen as a formidable voice in the area of education.

“The Badillo endorsement isn’t a huge surprise, but it works on a number of levels,” a Baruch College political science professor, Douglas Muzzio, said. “It has a demographic element, a partisan element, and an educational element. He is Puerto Rican, a former Democrat, and has an outstanding reputation in education. It is what you would expect Bloomberg would do. Bloomberg is on a shopping spree, and he’s buying up support.”

Mr. Badillo, for his part, said Mr. Bloomberg’s work in education reform is what made him decide to endorse him. “As far as I am concerned, education is the key to everything,” Mr. Badillo told the Sun. “You cannot have any real education unless you have standards and discipline, and by putting police in the schools, the mayor has really reduced violence in the schools.”

Mr. Badillo said he plans to campaign door-to-door with the mayor at the South Bronx, East Harlem, and Williamsburg. Mr. Badillo acknowledged that Mr. Bloomberg isn’t popular in those neighborhoods, but said: “All that can be overcome when they meet him. I was with the mayor at the Puerto Rican parade, and the way he was received there was very positive. I expect that will happen all over.”