By Thomas Ferraro

WASHINGTON (Reuters) - Several of President Barack Obama's Democrats in the U.S. Senate joined Republicans on Wednesday to block his nominee to a top Justice Department job after complaints that the lawyer, Debo Adegbile, had once helped represent a convicted "cop killer."

Adegbile, a former director at the Legal Defense Fund of the civil rights group NAACP, had been nominated to head the Justice Department's civil rights division.

Seven Democrats joined Republicans in the procedural vote blocking the nomination. Senate Majority Leader Harry Reid eventually joined them in a maneuver to preserve his right to bring up the nomination again.

The final vote was 47-52, with 51 votes needed for the nomination to proceed.

It was the first time that an Obama pick has been blocked in the Senate since Democrats changed the rules last year to strip Republicans of their power to stop a nominee on their own.

The Fraternal Order of Police had helped rally opposition against Adegbile, calling Obama's selection of him "a thumb in the eye of law enforcement."

Scores of civil rights groups backed Adegbile, calling him "a tireless advocate," and "a skilled litigator" who was well qualified for the job.

Backers said Adegbile should not be punished for his limited representation several years ago of Mumia Abdu-Jamal, who was sentenced to death for the 1981 murder of a Philadelphia police officer.

Abu-Jamal's case stirred debate inside and outside America about the fairness of the U.S. justice system and the application of the death penalty.

Courts upheld Abu-Jamal's conviction. Before Adegbile got involved, the sentence was reduced to life in prison because of what judges called improper instructions to the jury.

Senate Republican Leader Mitch McConnell opposed the nominee, saying, "Everyone deserves a fair trial and a zealous legal defense. And lawyers aren't personally responsible for the actions of their clients.

"But lawyers are responsible for their own actions. In this case, the nominee inserted his office in an effort to turn reality on its head, impugn honorable and selfless law enforcement officers, and glorify an unrepentant cop-killer," McConnell said.

Senate Judiciary Committee Chairman Patrick Leahy, a Vermont Democrat, defended the nominee, saying: "Debo's role in the Abu-Jamal case was limited to two Supreme Court briefs and one Third (U.S.) Circuit (Court of Appeals) brief."

"Attempts to attribute more to Debo, including out-of-court statements by other (NAACP) LDF attorneys, are unfounded," Leahy said.

(Reporting by Tom Ferraro and David Ingram; Editing by David Storey and Jonathan Oatis)