New York City Mayor Bill de Blasio Friday backed his former boss Hillary Rodham Clinton's White House bid after more than six months of withholding an endorsement in the race.

Appearing on MSNBC's "Morning Joe," the Democratic mayor said the former secretary of state, senator and first lady is best positioned to address issues such as the economy, campaign finance and criminal justice.

"The candidate who I believe can fundamentally address income inequality effectively, the candidate [who] has the right vision and the right experience and ability to get the job done, is Hillary Clinton," he said.

"She's very cold-eyed, and that's what you want in a president," he added. He said he will "absolutely" campaign for her.

On the day Clinton declared her candidacy in April, de Blasio went on NBC's "Meet the Press" to say he couldn't yet endorse her. Some Clinton allies criticized his hesitation.

De Blasio, who managed Clinton's successful 2000 race for the U.S. Senate, later suggested he wouldn't issue an endorsement until after a candidates' forum he is planning to host in December in Iowa. He had said he wanted to hear Clinton and other candidates spell out a progressive agenda. Clinton has not RSVP'd for the Iowa forum, nor have the other candidates.

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The Clinton campaign released a statement Friday trumpeting "over 135" mayors backing the candidacy. De Blasio is mentioned in a long list of other mayors, and was one of nine quoted in the news release.

"I'm honored to receive the support of more than 85 mayors across the country today -- because mayors know how to get things done," Clinton said in the statement.

De Blasio aide Phil Walzak declined to say whether his boss had met with Clinton in advance of the endorsement. Last month, de Blasio met quietly with Sen. Bernie Sanders (I-Vt.), Clinton's top Democratic primary rival.

When asked about Friday's endorsement of Clinton, a spokesman for Sanders pointed to de Blasio's comments about Sanders, whom he called an important progressive voice.

"The mayor's had very nice things to say about him, and we appreciate his kind words in the past," said Sanders spokesman Michael Briggs. De Blasio's endorsement, Briggs said, "went to his former boss."

Lis Smith, a spokeswoman for another Democratic opponent, former Maryland Gov. Martin O'Malley, declined to comment.

Christina Greer, a Fordham University political scientist, questioned the value of de Blasio's endorsement at this point in the race -- and whether de Blasio's opinion carries any weight in national politics.

"If a tree falls in the woods and no one's there to hear it, does it make a sound?" Greer said. "His endorsement is so late. It doesn't necessarily help Clinton with any particular demographic, and it doesn't give any value-added to de Blasio," she added.