SANTA FE, N.M. (AP) _ New Mexico Gov. Bill Richardson, the nation's only Hispanic governor, is endorsing Sen. Barack Obama for president, calling him a "once-in-a- lifetime leader" who can unite the nation and restore America's international leadership.

Richardson, who dropped out of the Democratic race in January, is to appear with Obama on Friday at a campaign event in Portland, Ore., The Associated Press has learned.

The governor's endorsement comes as Obama leads among delegates selected at primaries and caucuses but with national public opinion polling showing Sen. Hillary Rodham Clinton pulling ahead of him amid controversy over statements by his former pastor.

Richardson has been relentlessly wooed by Obama and Clinton for his endorsement. 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 has been mentioned as a potential running mate for either candidate.

No primaries are scheduled until Pennsylvania's on April 22, a gap in time Obama hopes to use for such announcements to assert that he is the front-runner for the nomination.

"I believe he is the kind of once-in-a-lifetime leader that can bring our nation together and restore America's moral leadership in the world," Richardson said in a statement obtained by the AP. "As a presidential candidate, I know full well Sen. Obama's unique moral ability to inspire the American people to confront our urgent challenges at home and abroad in a spirit of bipartisanship and reconciliation."

Richardson's endorsement also could help Obama pick up support among Hispanics, who are the nation's largest and fastest-growing minority.

Clinton has been the favorite of Hispanics in primaries and caucuses, according to exit polls. She won the New Mexico caucus in early February with a nearly 2-to-1 advantage among Hispanics.

Richardson backed Obama despite his ties to Clinton and her husband, the former president. He 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."

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. He showed this judgment by opposing the Iraq war from the start, and he has show it during this campaign by standing up for a new era in American leadership internationally," Richardson said.

Obama said he was "deeply honored" to have Richardson's support.

"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, earning five Nobel Peace Prize nominations," Obama said in a statement.