"I'm going to oppose him," Reid told reporters Thursday afternoon. "Here is a man who has been outspoken in denying equality for people to be married. He has been outspoken in trying to re-create the flag of the rebels — the Confederates.
"He's a person who, in my opinion, is not in the mainstream, and I don't think he deserves to be a federal judge."
When asked whether Boggs' nomination was in trouble, Reid said he would discuss the matter with his fellow Senate Democrats later in the day.
The majority leader stopped short of saying he wouldn't allow an up-or-down vote on Boggs if his nomination clears the Senate Judiciary Committee. "We'll see," Reid said.
The Georgia state judge has been picked by Obama to serve on a U.S. district court in the state. Some liberals, gay rights groups, civil rights leaders and a few congressional Democrats -- including members of the Congressional Black Caucus -- have opposed Boggs, a conservative Democrat.
Critics of the judge have complained about his past support for state proposals to post information online about doctors who perform abortions and to restore the Confederate battle emblem on the Georgia flag.
Boggs is one of several nominees for federal court posts in Georgia — a compromise deal worked out between the White House and Georgia lawmakers.
While testifying before the Senate judiciary panel on Tuesday, Boggs said that as a state lawmaker, he was representing his constituents' views. He said he now believes his vote on listing the names of abortion doctors online was wrong and he is glad the Confederate emblem was later removed from the state flag.
Earlier in the week, White House spokesman Jay Carney defended Boggs, saying that "of all the recent criticisms offered against Michael Boggs, not one is based on his record as a judge for the past 10 years" on a state appeals court.
Information from the Associated Press was used in this report.