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Senate Democrats failed to muster enough support to push a controversial Justice Department nominee past a key test vote on Wednesday, leaving the nomination stalled amid bipartisan concerns about his past work on behalf of convicted cop killer Mumia Abu-Jamal.

The nomination of Debo Adegbile for head of the Civil Rights Division was blocked, on a 47-52 vote.

Adegbile needed just 51 senators to support him in order to advance to a final vote. In anticipation of a tight vote, Vice President Biden even attended in case he was needed by his Democratic colleagues to break a tie. But it did not get to that point, as a total of seven Democrats peeled off, joining a nearly united Republican opposition.

One of them, Indiana Sen. Joe Donnelly, said afterward he was "concerned about his ability to work with law enforcement" if confirmed.

Maureen Faulkner, widow of the Philadelphia officer killed by Abu-Jamal, told Fox News after the vote that Adegbile was "the wrong person for the job." She thanked Democratic senators who "broke ranks and had the courage to do the right thing."

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    Democrats were facing heavy pressure from police organizations, as well as Republicans, to vote against the nominee. After the tally, Republican National Committee Chairman Reince Priebus called the nomination itself "an embarrassment for President Obama," and blasted "vulnerable Democrats running in 2014" who voted "yes."

    Lawmakers also faced pressure from Faulkner, who Pennsylvania Democrat Sen. Bob Casey and other senators who were "sitting on the fence," urging them to reject Adegbile's nomination.

    Obama, though, stood by his nominee and called the outcome a "travesty based on wildly unfair character attacks against a good and qualified public servant."

    In a written statement, Obama said: "The fact that his nomination was defeated solely based on his legal representation of a defendant runs contrary to a fundamental principle of our system of justice -- and those who voted against his nomination denied the American people an outstanding public servant."

    Sen. Casey, a high-profile Democrat, effectively gave his Democratic colleagues cover to vote "no" after he recently came out against the nomination. Casey represents the state where Abu-Jamal killed a Philadelphia police officer.

    At issue is Adegbile's work with the NAACP Legal Defense Fund on behalf of Abu-Jamal. Perhaps America's most well-known cop killer, he was convicted for the 1981 murder of Philadelphia officer Daniel Faulkner. The case has attracted celebrity and media attention for decades, amid claims that Abu-Jamal faced racial discrimination. Abu-Jamal's death sentence was eventually overturned, but his conviction stands.

    Along the way, Adegbile and the Legal Defense Fund got involved in the case, filing briefs and appeals on his behalf. Adegbile's involvement came after Abu-Jamal's death sentence was overturned and after the Legal Defense Fund first began working with the defendant.

    Adegbile's defenders argue that he was obligated to work on the case since the Legal Defense Fund had already accepted him as a client. And they argue that defending unpopular clients is just part of the legal profession.

    "This murderer's a bad guy, but he's entitled to a lawyer," Senate Majority Leader Harry Reid, D-Nev., said on the Senate floor before the test vote. He stressed that Adegbile never stepped "into a courtroom" to defend Abu-Jamal, and called the nominee a "fine man" who "devoted his life to public service." (Reid later joined the other seven Democrats in voting "no," but only for procedural reasons.)

    But critics claimed Adegbile took on the case as a political cause.

    Republicans and law enforcement groups like the Fraternal Order of Police were staunchly opposed. Sen. Jeff Sessions, R-Ala., questioned Tuesday whether Adegbile could be counted on to advocate for the civil rights "of all Americans," if confirmed to the post of Civil Rights Division director.

    The president of the Fraternal Order of Police has called the nomination a "thumb in the eye of our nation's law enforcement officers."

    Faulkner's widow has adamantly opposed the nomination and lobbied senators to vote "no."

    Both of Pennsylvania's senators, Casey and Republican Sen. Pat Toomey, earlier announced their opposition.

    "I respect that our system of law ensures the right of all citizens to legal representation no matter how heinous the crime," Casey said in a statement.

    "At the same time, it is important that we ensure that Pennsylvanians and citizens across the country have full confidence in their public representatives -- both elected and appointed. The vicious murder of Officer Faulkner in the line of duty and the events that followed in the 30 years since his death have left open wounds for Maureen Faulkner and her family as well as the city of Philadelphia. After carefully considering this nomination and having met with both Mr. Adegbile as well as the Fraternal Order of Police, I will not vote to confirm the nominee."

    Obama nominated Adegbile to succeed Thomas Perez in the DOJ post, after Perez was confirmed for Labor secretary.

    The other Democrats who voted against Adegbile are: Sens. Chris Coons, of Delaware; Heidi Heitkamp, of North Dakota; Joe Manchin, of West Virginia; Mark Pryor, of Arkansas; and John Walsh, of Montana.

    Adegbile is the first Obama nominee to fail on a Senate test vote since Democrats pushed through changes that allow certain nominees to clear that hurdle with a simple majority, as opposed to the customary 60 votes.

    Fox News' Chad Pergram and Donald Fair contributed to this report.