(CNN) -- Declaring that Sen. Barack Obama is an "extraordinary American," Gov. Bill Richardson of New Mexico endorsed Obama for the Democratic nominee for president on Friday.
Richardson -- who sought this year's Democratic nomination for president himself -- joined Obama at a rally in Portland, Oregon, where the senator from Illinois is campaigning.
"Barack Obama will make a great and historic president," Richardson said, Obama standing at his side. "[It] is a once-in-a-lifetime opportunity for our nation and you are a once-in-a-lifetime leader."
"He's done the kind of work that you want from your public servants, somebody who's driven not just by raw ambition, not just by an interest in personal aggrandizement," Obama added. "He's been somebody who's been motivated by the desire to make the lives of his constituents and working people a little bit better."
Richardson praised Obama for his speech this week on race in America, saying "he appealed to the best in us." Watch Obama's speech on race »
"As a Hispanic-American, I was particularly touched by his words," Richardson said, putting his arm around Obama and declaring in Spanish that he is "a man who understands us."
Richardson is the nation's only Hispanic governor. Hispanics have tended to support Sen. Hillary Clinton in her quest for the Democratic nomination.
Obama and Clinton both lobbied Richardson for his endorsement after he dropped out of the race January 10. Richardson called Clinton Thursday to tell her of his decision, Clinton's campaign said.
The campaign shrugged off Richardson's endorsement of her rival.
"Both candidates have many great endorsers, but the voters, not endorsers, will decide this election, and there are still millions of voters in upcoming contests who want to have their voices heard," Clinton spokesman Jay Carson said.
Richardson was secretary of energy and U.S. ambassador to the United Nations under President Bill Clinton. He said he remains friends with the Clintons, and watched the Super Bowl with Bill Clinton this year.
Richardson's endorsement may be more important for its influence on superdelegates, the nearly 800 Democratic party officials whose backing will be essential for either candidate to win the party's nomination, according to CNN's senior political correspondent, Candy Crowley.
As a governor, Richardson is a superdelegate.
"It is time ... for Democrats to stop fighting amongst ourselves and to prepare for the tough fight we will face against John McCain in the fall," Richardson said, referring to the probable Republican nominee.
The Clinton and Obama campaigns have been waging an intense battle for the backing of superdelegates, roughly half of whom have yet to declare their support.
"This is a larger message to superdelegates, those elected officials and party officials who, in the end, may well decide who the nominee will be," Crowley said.
Richardson pointed out in his speech that Obama is "after all, well ahead in the delegate race for our party's nomination," drawing a roar of approval from the crowd of Obama supporters.
Obama leads Clinton by 137 delegates, according to the latest CNN count.
Richardson said his "affection for Hillary Clinton and President Bill Clinton will never waver" but that "it is now time for a new generation of leadership."
Obama "can bring us the change we so desperately need by bringing us together as a nation here at home and with our allies abroad," he said.
He drew laughs with a story about how Obama had bailed him out when a moderator called on him unexpectedly during a Democratic debate.
"I was about to ask the moderator to repeat the question when Barack whispered to me 'Katrina, Katrina.' And I gave my Katrina answer. He could have thrown me under the bus, but he stood behind me."
Obama had earlier praised Richardson.
"Whether it's fighting to end the Iraq war or stop the genocide in Darfur or prevent nuclear weapons from falling into the hands of terrorists, Gov. Richardson has been a powerful voice on issues of global security, peace and justice," Obama said in a statement released before the endorsement.
Richardson is the second former Democratic presidential contender to endorse Obama, after Sen. Chris Dodd of Connecticut. Two other former candidates, Sen. Joe Biden of Delaware and former Sen. John Edwards of North Carolina, have remained neutral.
None of the dropouts has endorsed Clinton. E-mail to a friend