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Gov. Bill Richardson (D-NM) attends a performance by recording artists Vince Gill and Amy Grant at the White House in Washington on February 24, 2008. (UPI Photo/Kevin Dietsch)

"Your candidacy is a one-in-a lifetime opportunity for our nation and you are a once-in-a-lifetime leader," Richardson, who left the Democratic nomination race in January, said during a Portland, Ore., rally.

Obama's speech on race "started a discussion in this country that is long overdue," Richardson said, adding he was "particularly touched" as a Hispanic-American.

"(You) will be a president who will bring this nation together and restore America's global leadership," he said as chants of "Yes we can" filled the auditorium. "I am very proud to endorse your candidacy for president."

Richardson, a member of the Bill Clinton administration, said Democrats were "blessed to have two great American leaders" in Obama, D-Ill., and Sen. Hillary Clinton, D-N.Y., running for president.

Expressing affection for the Clintons, Richardson said, "It's time, however, for Democrats to stop fighting amongst ourselves and prepare for the tough fight" against Sen. John McCain, R-Ariz., the presumptive Republican presidential nominee.

Clinton's campaign said she and Obama "have many great endorsers" but "voters, not the endorsers, will decide the election."