PORTLAND, Ore. — The nation’s only Hispanic governor, Bill Richardson, threw his support behind Senator Obama for president today, delivering one of the most coveted and tightly held endorsements in the race for the Democratic nomination.

The New Mexico governor joined Mr. Obama at a spirited rally today and said the Illinois senator demonstrated his leadership abilities this week with his speech on race. “You are a once-in-a-lifetime leader,” the governor said from the stage. “Above all, you will be a president who brings this nation together.”

Mr. Richardson dropped his own bid for the nomination in January. His support for Mr. Obama comes during a tough period for the senator, the leader in the delegate chase over Senator Clinton. Mr. Obama has seen his lead in national polls wither as he’s grappled with the fallout from divisive remarks by his former pastor.

Mr. Richardson was relentlessly courted by both candidates and his support for Mr. Obama represents a potential counterweight to Mrs. Clinton’s strength among Hispanic voters.

As a Democratic superdelegate, the governor plays a part in the tight race for nominating votes and could bring other superdelegates to Mr. 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 Mr. Obama hopes to use for such announcements to assert that he is the front-runner for the nomination.

Mr. Richardson backed Mr. Obama despite his ties to Mrs. Clinton and her husband, the former president. Mr. Richardson served as ambassador to the United Nations and as secretary of the Energy Department during the Clinton administration. Last month, Mr. Richardson and President Clinton watched the Super Bowl together at the governor’s residence in Santa Fe.

Mr. Richardson praised Mrs. Clinton as a “distinguished leader with vast experience.” But the governor said Mr. 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 Mrs. Clinton’s victory in New Mexico in February, a senior strategist, Mark Penn, said, “Perhaps the time when he could have been most effective has long since past.”

Mr. Richardson was a roving diplomatic troubleshooter when he was a congressman from New Mexico, negotiating the release of American hostages in several countries and meeting with a rogue’s gallery of American 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,” Mr. 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.”

Mr. Obama said “I can’t be more honored” to have Mr. Richardson’s support.