Sen. Tom Harkin, D-Iowa, accused the Senate of setting a double standard for black nominees after it rejected an Obama administration pick for a top Justice Department post.
Harkin said the vote Wednesday to reject Debo Adegbile as head of the department's civil rights division is one of the most "shameful" he's witnessed in his 30 years.
The Senate voted 52-47 to block Adegbile's nomination, with seven Democrats joining the Republicans in opposition.
Critics of the nomination, such as the Fraternal Order of Police, had objected because of Agdebile's role in assisting legal appeals for a man convicted of the 1981 killing a Philadelphia police officer, Danny Faulkner.
The man, Mumia Abu Jamal, was sentenced to death, and Adegibile was among a group of lawyers at the NAACP Legal Defense Fund who in 2006 appealed his death sentence.
The U.S. Supreme Court ordered a new sentencing hearing, but prosecutors decided not to move ahead, and Abu Jamal is serving a life sentence.
In a floor speech Wednesday and in a conference call with Iowa reporters Thursday, Harkin said Adegbile played only a small part in the appeal. And he complained that Supreme Court Chief Justice John Roberts, who he noted is white, had once been assigned to give pro bono time to assist in the case of a Florida man convicted of several murders in Florida.
Harkin said he didn't fault Roberts for the work. But, he added, "If you're a young black person and you work for the NAACP Legal Defense Fund and you're asked to sign an appeal for someone convicted of murder, the message sent today is, 'Don't do it.'"
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"We sent a message that we have a double standard, a terrible double standard," Harkin said.
Some Republicans said Agdebile was too ideological, but the murder case played a big role.
Sen. Pat Toomey, R-Pa., read a letter from Faulkner's widow, saying it would be "revolting" that Agdegbile would be rewarded after assisting her husband's killer.
Asked whether he thought race motivated senators, Harkin said they were afraid of television ads that might be aired criticizing their vote if they approved.