(CNN) – Bill Richardson criticized a Clinton campaign adviser Friday for suggesting his endorsement of Barack Obama is insignificant.
"I resent the fact that the Clinton people are now saying that my endorsement is too late because I only can help with Texans - with Texas and Hispanics, implying that that's my only value," the New Mexico governor told CNN's John King.
"That's typical of some of his advisers that kind of turned me off."Earlier Friday, Clinton campaign senior strategist Mark Penn said he thought Richardson's endorsement came too late to make an impact.
“The time that he could have been effective has long since passed," Penn told reporters on a conference call. "I don’t think it is a significant endorsement in this environment.”
In the interview Friday, Richardson also said he called Hillary Clinton Thursday to inform her of his decision to back Obama, a conversation he described as "painful."
"It was painful and it wasn't easy," he said. "I've spoken to others who have had that same conversation and they say at the end, it’s not all that pleasant.
"The former Democratic presidential candidate declined to elaborate further on his conversation with Clinton.Last month, Chris Dodd - another former presidential candidate who decided to endorse Obama last month - said he had a "not comfortable" conversation with Clinton informing her of the news.
Also in the interview Friday, Richardson said he ultimately decided to back Obama because the Illinois senator has "something special."
"I think that Sen. Obama has something special,” explained Richardson. “Something that can bring internationally America’s prestige back, that can deal with the race issue as he did so eloquently last week, that can deal with the domestic issues in a bipartisan way."
Richardson, who held posts as the Secretary of Energy and the U.S. Ambassador to the U.N. in President Clinton’s administration, also said he "owes a lot to the Clinton family but I served well. I paid it back in service to the country."
Related video: Richardson on Obama endorsement
Updated 7:22 p.m. to add link to interview video
– CNN's Martina Stewart and Alex Mooney