(CNN) -- Sen. Barack Obama's speech on race was a decisive factor in Bill Richardson's endorsement of his presidential bid, the New Mexico governor revealed Saturday.

"I think what kind of clinched it for me, although I made a decision a week ago, was Sen. Obama's speech on race," Richardson said on ABC's "Good Morning America."

"He had this problem with his pastor. He could have said nothing or glided through it. Instead, he attacked the race issue head-on, talking about stereotypes, taking some very, very tough stances on this issue."

Richardson, who had sought this year's Democratic nomination for president himself, was referring to controversy surrounding racially charged comments made by the Rev. Jeremiah Wright, Obama's former pastor.

In his Tuesday speech, Obama denounced some of Wright's comments but said he could not repudiate the man himself.

On Friday at a rally in Portland, Oregon, with Obama, Richardson praised the Democrat from Illinois for the speech, saying "he appealed to the best in us." Watch Richardson call Obama a 'once-in-a-lifetime leader' »

Richardson, a former Clinton cabinet official, had been undecided for weeks after ending his own presidential run.

He said Saturday that he admired both former President Clinton and Sen. Hillary Clinton but that "there were a couple of times when I thought they got too negative, though."

Another former Clinton White House aide, political strategist James Carville -- who is also a CNN political analyst -- told the New York Times in a story published Saturday that Richardson's decision was an "act of betrayal."

The endorsement "came right around the anniversary of the day when Judas sold out for 30 pieces of silver, so I think the timing is appropriate, if ironic," Carville said, referring to the Easter weekend.

On a Saturday conference call, Clinton spokesman Howard Wolfson said he did not share that assessment, telling reporters that "I don't think it's a question of betrayal. Anyone is free to endorse whoever they want for whatever reasons they think are appropriate."

He also dismissed the idea that Richardson's decision might sway the superdelegate votes crucial to Clinton's presidential hopes.

Wolfson said the former United Nations ambassador will not be in a position to argue that superdelegates should abide by the will of the voters, since Clinton won his home state of New Mexico.

As a governor, Richardson is a superdelegate. E-mail to a friend

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