The endorsement in Oregon kicked off a two-day campaign swing for Obama. He scheduled stops in Salem, Eugene and Medford after the Portland appearance, which drew a crowd that packed the Memorial Coliseum. Supporters grabbed the 12,000 tickets within hours after they were available online and at Portland campaign headquarters.
Richardson dropped his own bid for the nomination in January. Richardson was relentlessly courted by both candidates and his support for Obama represents a potential counterweight to Clinton's strength among Hispanic voters.
It wasn't the first time racial concerns had helped to drive a prominent backer to Obama. Massachusetts Sen. Edward Kennedy was moved to drop his neutrality and side with Obama in part because of what he saw as Bill Clinton's racially tinged criticisms of the senator.
Richardson backed Obama despite his earlier statements that superdelegates, of which he's one, should pick sides based on the votes of their state or constituency. By that reasoning, he might have been expected to support Clinton because she won the New Mexico contest.
As a Democratic superdelegate, the governor plays a part in the tight race for nominating votes and could bring other superdelegates to Obama's side. He also had been mentioned as a potential running mate for either candidate.
No primaries are scheduled until Pennsylvania's on April 22, a gap Obama hopes to use for such announcements to assert that he is the front-runner for the nomination. Oregonians begin primary voting in early May, and the ballots are counted May 20.
Richardson served as ambassador to the U.N. and as secretary of the Energy Department during the Clinton administration. Last month, Richardson and former President Clinton watched the Super Bowl together at the governor's residence in Santa Fe.
Richardson praised Hillary Clinton as a "distinguished leader with vast experience." But the governor said Obama "will be a historic and great president, who can bring us the change we so desperately need by bringing us together as a nation here at home and with our allies abroad."
The Clinton campaign was publicly dismissive of the endorsement, after the New York senator failed to win it for herself.
Citing Clinton's victory in New Mexico in February, senior strategist Mark Penn said, "Perhaps the time when he could have been most effective has long since past."
Richardson was a roving diplomatic troubleshooter when he was a congressman from New Mexico, negotiating the release of U.S. hostages in several countries and meeting with a rogue's gallery of U.S. adversaries, including Saddam Hussein and Fidel Castro.
"There is no doubt in my mind that Barack Obama has the judgment and courage we need in a commander in chief when our nation's security is on the line," Richardson said. "He showed this judgment by opposing the Iraq war from the start, and he has shown it during this campaign by standing up for a new era in American leadership internationally."
Obama said "I can't be more honored" to have Richardson's support.
The Associated Press contributed to this report.