Seven Democrats joined Senate Republicans to block the nomination of a Senate Judiciary Committee staffer and civil rights lawyer whom President Obama had selected to run the Justice Department's civil rights division.

Vice President Joe Biden was on hand to cast a tie-breaking vote in favor of Debo Adegbile if it became necessary, but Adegbile's history as an attorney -- most notably, his decision to revive the defense of a man convicted of murdering a police officer -- rendered his support irrelevant. Adegbile is the first nominee to have his nomination blocked since Senate Majority Leader Harry Reid, D-Nev., pulled the trigger on the "nuclear option" rule change.

Adegbile's involvement with the NAACP's Legal Defense Fund, which successfully overturned a death sentence for Mumia Abu Jabal on the theory that he was a victim of a racist justice system, proved toxic to his nomination.

“This is not a matter of left-wing or right-wing politics,” Sen. Ted Cruz, R-Texas, said on the Senate floor Tuesday. “We all should agree that violent criminals should be punished. We should all agree that those who advocate for, celebrate, lionize convicted cop killers are not suitable for major leadership roles at the United States Department of Justice. I urge every member of this body to oppose this nomination.”

Senate Minority Leader Mitch McConnell, R-Ky., took to the floor Wednesday to make the case against Adegbile, focusing on the Abu Jamal case and other issues, such as when Adegbile described voter ID laws as a poll tax.

"And in Hosanna-Tabor v. EEOC, he took the position in the Supreme Court that a church did not have the First Amendment right to hire or fire individuals who were responsible for conveying the church's message and implementing its mission," McConnell said. "The position the nominee advocated would greatly infringe on the Free Exercise rights of religious institutions. The Supreme Court rejected his views there, too--this time unanimously, 9-0."

Biden's replacement, Sen. Chris Coons, D-Del., voted against Adegbile. So did Sen. Bob Casey, D-Penn., Sen. Joe Donnelly, D-Ind., Sen. Heidi Heitkamp, D-N.D., Sen. Joe Manchin, D-W.Va., Sen. Mark Pryor, D-Ark., and Sen. John Walsh, D-Mont. -- all of whom either face tough GOP challenges in 2014 or potentially could in later years.

Reid changed his vote to "no," bringing the official tally to 47-52, to ensure he can seek a reconsideration of the vote if he can get any opponents of the nomination to change their minds.

The blocking of Adegbile marks the first time Senate Democrats have failed to advance one of President Obama's nominees since Reid pulled the trigger on the nuclear option. Under the new rules, Republicans can't filibuster executive branch nominees, allowing Democrats to rubber-stamp them.

In the case of Adegbile, though, Reid couldn't keep Democrats united.