Richardson Backs Obama, Rebuffs Clinton

"Coming off the fence shows other superdelegates that you can go against the Clintons even if they've helped you become a national figure or even if they've helped you in your career," ABC News political consultant Mark Halperin said of the endorsement.

Fierce Fight as Obama Camp Gets Tough

Senator Obama, for his part, stayed above the fray, leaving the fierce Democratic party in-fighting to his campaign manager, who earlier in the day had assailed Hillary Clinton's "character gap".

"She is not seen as trustworthy by the American people. She has consistently, in this campaign, engaged in political miscalculation to mislead voters," David Plouffe, Obama's campaign manager said on a conference call with reporters on Friday.

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Obama Camp: Clinton Has 'Character Gap'

The Clinton campaign responded in a call of its own.

"The Obama campaign is in political hot water given the news stories of the last couple weeks and is basically desperate to change the subject," fired back Clinton campaign spokesperson Phil Singer.

The nastiness between Obama and Clinton is having an impact. Polls indicate a sizeable minority of Clinton supporters have developed resentments toward Obama and vice versa.

"Intensity matters and it just takes a few percentage of voters to sit on their hands, stay at home, and not vote at all or switch sides and vote for John McCain and that could cost you a swing state like Ohio or even Pennsylvania," said Jay Carney, deputy bureau chief for TIME magazine in Washington.

Hispanic Guv Could Help With Key Group

Richardson, the nation's only Hispanic governor, has been actively courted by both campaigns since he abandoned his own bid for the White House in January, after poor showings in both the Iowa caucuses and New Hampshire primary.

In his endorsement, Richardson called Obama a friend, noting such personal similarities as their mixed ethnic identity. Both he and Obama have one foreign-born parent. Richardson's mother was from Mexico, and Obama's father was from Kenya.

Richardson also said Obama's race speech resonated with him as an ethnic minority.

"As a Hispanic, I was particularly touched by his words. I have been troubled by the demonization of immigrants -- specifically Hispanics -- by too many in this country," he said.

Richardson also recounted a story from the days when he was still a presidential candidate.

He recalled that during one debate, he was not paying attention to a question asked of him by the moderator. Obama, then still a rival for the nomination, was seated next to him and whispered, "Katrina, Katrina."

"He could have thrown me under the bus," Richardson laughed. "But he stood behind me."

Obama's Communications Director Robert Gibbs said that Obama spoke with Richardson weekly over the telephone, but while visiting Richardson's home state last month, Obama told reporters that he was not necessarily expecting the governor's endorsement.