Governor-elect Spitzer’s decision to bring on a Republican state senator as his homeland security chief has other Republicans concerned about the party’s slim margin in the Senate.

Mr. Spitzer announced yesterday that he was tapping Senator Michael Balboni, who represents Long Island, as his deputy secretary of public safety, a Cabinet-level position that includes overseeing emergency preparedness, law enforcement, and all other homeland security matters.

“Since September 11, he has been at the forefront of the state’s homeland security and disaster preparedness,” Mr. Spitzer said of Mr. Balboni, who has been chairman of the Committee on Veterans, Homeland Security, and Military Affairs.

The move is a politically shrewd one for Mr. Spitzer, a Democrat. Not only does it help him burnish credentials as a bipartisan lawmaker willing to go beyond party loyalists to fill a key senior position, it could also give his party a chance to further chip away at the Republicans’ eightseat majority in the Senate.

Mr. Spitzer will get to call a special election to fill Mr. Balboni’s seat in Nassau County, which has more registered Democrats than Republicans.

The appointment comes at a tumultuous time for the New York State Republican Party. Not only is the GOP losing its hold on the governor’s mansion, but the majority leader in the senate, Joseph Bruno, is the subject of a federal investigation involving his business dealings.

In Albany yesterday, Mr. Spitzer said the move was “not part of any other ulterior scheme.”

Mr. Balboni’s decision to accept a position in Mr. Spitzer’s Democratic administration is a blow to Mr. Bruno and the recently elected Republican state party chairman, Joseph Mondello, who shut down Mr. Balboni’s efforts to run for attorney general last year for fear that his seat was vulnerable to a Democratic pickup. While Mr. Balboni pledged his support for Mr. Bruno yesterday, the election to replace him could increase pressure on the majority leader to step down from his leadership position, given the FBI investigation.

Mr. Balboni, who was elected to the state Senate in a 1997 special election, said yesterday that Mr. Bruno was “gracious” when told about the new job. “He said to me, ‘You can’t pass up this opportunity,'” Mr. Balboni said, according to the Associated Press.

A Republican state senator from Brooklyn, Martin Golden, said it was a “little bit of shock” that Mr. Balboni chose to take the job.

“It obviously presents challenges going into the new year that we didn’t have,” Mr. Golden said. “It’s going to be costly, and timely, and it’s going to be quite the challenge to be able to win that. But, I do believe at the end of the day we will be successful.”