WASHINGTON — U.S. Sen. Mark Pryor has become just the fifth Democrat to vote against one of President Barack Obama’s judicial nominees.

The Arkansan who is facing re-election in 2014 joined two other Democrats early Thursday to vote against confirming Cornelia "Nina" Pillard for the U.S. Court of Appeals for the District of Columbia Circuit.

Pillard, a Georgetown University Law professor who has argued nine cases before the U.S. Supreme Court, narrowly won confirmation 51 to 44 with no Republican in favor.

Conservatives have been outspoken in criticizing Pillard’s nomination to serve on a court that handles the bulk of cases filed against federal regulations. Iowa Sen. Charles Grassley, the ranking Republican on the Judiciary Committee, raised questions about her views on contraception, reproductive rights and gender equality.

Pryor said he voted against Pillard because he questioned whether she could be fair and impartial. He has otherwise voted in favor of 103 other judicial nominees that President Obama has offered since taking office in January 2009.

Rep. Tom Cotton, R-Dardanelle, who is challenging Pryor in 2014, has complained that the two-term senator too often toes the party line rather than the values of Arkansans — repeating a common refrain that Pryor votes with Obama 95 percent of the time.

Pryor argues that he is a moderate who is willing to bolt from the party line on issues important to Arkansas, such as gun ownership.

Senate Democrats voting against Pillard were Pryor, Sen. Joe Manchin of West Virginia and Sen. Joe Donnelly of Indiana. The roll call vote came just before 1 a.m. Thursday as the Senate pulled its first of two all-nighters to consider a backlog of nominations.

The Senate last week changed its rules — over the opposition of the Republican minority — to make it easier for nominations to be brought to a vote. The rules now allow a simple majority to bring debate to a close rather than the 60-vote majority previously required.

Only four of the 103 other judicial nominees confirmed by the Senate since Obama took office have had Democratic opposition. Oregon Sen. Jeff Merkley voted against one and former Nebraska Sen. Ben Nelson voted against three.