Queens Democrats took a step on Friday toward unifying their party in an effort to unseat State Senator Serphin R. Maltese, a Republican, when a candidate withdrew from the Democratic primary, leaving one challenger to the longtime incumbent.

Albert J. Baldeo, a lawyer who ran against Mr. Maltese two years ago, announced at a news conference that he was taking himself out of the race and endorsing the remaining Democratic candidate, City Councilman Joseph P. Addabbo Jr.

Mr. Maltese, who has represented the 15th District since 1988, has been deemed vulnerable by Senate Democratic leaders in their quest to regain control of the Senate, which the Republicans hold by a one-seat majority.

Two years ago, Mr. Baldeo was a little-known lawyer in Queens who came within 2 percentage points, about 900 votes, of defeating Mr. Maltese. And he achieved that with no support from the party organization in Queens and largely with his own money.

This year, too, Mr. Baldeo’s desire for a rematch never acquired much support from Queens Democratic officials. They preferred Mr. Addabbo, the son of a well-known congressman.

In an interview on Friday, Mr. Baldeo said he had become increasingly aware that the party would have a better chance to topple Mr. Maltese if a divisive Democratic primary could be avoided.

“It was a gut-wrenching, difficult decision,” Mr. Baldeo said. “But I decided to endorse him for the greater goal of Democratic Party unity. We want to put all of our resources behind one candidate, and that’s Councilman Addabbo. I’ll be actively campaigning for him.”

The two men appeared together at the news conference, at Mr. Baldeo’s office in Richmond Hill, Queens. The event even attracted a few of Mr. Maltese’s aides, who attended with video cameras.

Mr. Baldeo has had an arms-length relationship with Queens Democrats for some time. And he has been described by party leaders as something of an unpredictable candidate. Despite Mr. Baldeo’s strong showing in his previous contest with Senator Maltese, party leaders made their preference for Mr. Addabbo clear long before the councilman entered the race officially.

When asked whether he had been pressured to leave the race, either by Representative Joseph Crowley, the Queens Democratic leader, or Michael H. Reich, the organization’s executive secretary, Mr. Baldeo said only that he had been “in consultations with various party leaders,” and declined to be more specific.

“The important thing is that we are now united behind a candidate in the Democratic primary,” he said.

Despite the withdrawal, Mr. Baldeo’s name will remain on the ballot for the Sept. 9 primary. On Friday, Mr. Addabbo called Mr. Baldeo “a worthy opponent.”

He added: “I applaud his wholehearted embrace of the democratic process. The Democratic Party is the party of inclusion; it’s a big tent with room for everyone, and we are stronger for his efforts.”

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