And Obama has seen his lead in national polls wither in the fallout from divisive remarks by his former pastor.
Clinton, encouraged by polls forecasting a big but expected win in Pennsylvania next month, shows no signs of giving up.
The Clinton campaign publicly dismissed Richardson’s endorsement, with senior strategist Mark Penn saying: “Perhaps the time when he could have been most effective has long since passed.”
Pressure is mounting on the Democrats to rally behind a single candidate so that he or she can focus on battling the presumptive Republican nominee, John McCain, for the White House.
McCain, who has had the luxury of leaving behind party skirmishes since his last rival, Mike Huckabee dropped out on March 4, has been touring the world to enhance his foreign policy resume.
The race between Clinton and Obama has become fiercer and more bitter.
A retired general and Obama backer last week equated comments by Bill Clinton to the anti-communist witch hunts of the 1950s.
Retired General Merrill “Tony” McPeak, a co-chairman of Obama’s campaign, accused the former president of being divisive and trying to question Obama’s patriotism when he spoke of an election pitting his wife against McCain.
Bill Clinton had told a group of veterans in North Carolina on Friday: “I think it would be a great thing if we had an election year where you had two people who loved this country and were devoted to the interest of this country.
“And people could actually ask themselves who is right on these issues, instead of all this other stuff that always seems to intrude itself on our politics.”
McPeak on Friday compared the former president’s comments with the actions of Joseph McCarthy, the 1950s communist-hunting senator, saying he had had enough of people “accusing good Americans of being traitors”.
Richardson, a “super delegate” and former president Bill Clinton’s ambassador to the UN, said that to beat McCain, the Democrats need to unite now – behind Obama.
“We need to end this, we need to get ready for November, we need to be positive, we have to stop these personal attacks – they’re reaching excessive amounts,” he said.
Richardson, the only Hispanic governor in the US, endorsed Obama for president on Friday, saying the Illinois senator demonstrated his leadership abilities last week with his speech on race.
“You are a once-in-a-lifetime leader,” the governor said at a rally. “Above all, you will be a president who brings this nation together.”
Richardson dropped his own bid for the nomination in January.