By GREG WILSON

Daily News Staff Writern his bid for borough president, State Sen. Pedro Espada Jr. turned an opponent into a supporter yesterday by picking up the endorsement of salsa singer Willie Colon.

Colon had been campaigning for the job himself, but changed his tune yesterday and backed the Bronx Democrat. He said he will instead mount a campaign for public advocate. Espada said he will support Colon's new bid.

"After careful thought and consideration, I have determined that Sen. Espada has the experience, track record and vision to lead 1.3 million Bronxites into an era of positive change and progress," Colon said.

Espada said the two reached an agreement after 10 days of negotiations, which culminated in a marathon discussion of issues earlier this week.

"We were up until 2:30 a.m., talking about our positions on key issues," Espada said. "We needed to know where each other stood, and how we could work together."

With Colon dropping out, the field shrunk to four announced candidates. City Councilman Larry Warden (D-Northeast Bronx) is said to be leaning toward running, so the number could return to five.

The Bronx Democratic organization is backing West Bronx Councilman Adolfo Carrion. A breakaway group of Democrats, from the Ben Franklin Democratic Club in Riverdale, is backing City Councilwoman June Eisland. Ronald Law, a onetime aide to Sen. Daniel Moynihan and to former Deputy Mayor Rudy Washington, also has announced plans to run.

Espada has feuded with the local party hierarchy for years. Last year, he was acquitted of larceny charges for allegedly using money from his health care business to fund his campaigns. He has blamed party leaders for the prosecution.

Espada won back the 32nd District Senate seat he held from 1992 to 1996 in November, even as he stood trial, after defeating incumbent David Rosado in a Democratic primary.

Colon, a famous salsa musician, first ran for office in 1994 by challenging Rep. Eliot Engel (D-Bronx, Westchester).