Richardson's backing of Obama, who aspires to become America's first black president, could show most dramatically among the governor's fellow Hispanics -- the US' largest ethnic bloc, which has largely backed Clinton.
He told Democrats it was time to stop bickering and get behind Obama, but the Clinton campaign dismissed the endorsement.
Senior strategist Mark Penn, noting Clinton's February victory in the New Mexico primary, said, "Perhaps the time when he could have been most effective has long since past."
Penn added that he didn't think it was a "significant endorsement."
The nod from Richardson had been sought relentlessly by both Obama and Clinton. Bill Clinton even went to Richardson's New Mexico home in January to watch the US' premier television sporting event, the football Super Bowl.
As a Democratic superdelegate, Richardson has a key role in the tight race for nominating votes and could bring other superdelegates to Obama's side. He also has been mentioned as a potential running mate for either candidate.
Obama leads Clinton among delegates whose votes were determined by primaries or caucuses, 1,406 to 1,249. But neither is on track to win enough pledged delegates to clinch the nomination -- 2,024 are needed. So the outcome could be decided by superdelegates -- elected and party officials who can choose whomever they like.
Including Richardson, Clinton leads 250 to 215 among superdelegates who have announced a choice. About 40 percent have not declared.
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