Bernie Sanders Rejects Martin O’Malley's Attacks as 'Categorically False' The Vermont senator says he always backed President Obama.

Sanders rejected O'Malley's claims on ABC's "This Week," saying Obama campaigned for him in Vermont in 2006 and that he worked for the president in 2008 and 2012. O'Malley had said Friday that in 2011, Sanders was trying to find someone to run against the president in a primary.

"I worked very hard to see Barack Obama elected," Sanders said. "I think under incredible Republican obstructionism, Obama and Joe Biden have moved this country forward in a way that leaves us a hell of a lot better than we were when [former President George W.] Bush left office."

O'Malley's accusations stem from Sanders saying at least once in 2011, during a radio interview, that he was in favor of robust primaries.

Sanders told Stephanopoulos, “Somebody asked me years ago, do you think there should be a primary opponent to Barack Obama? And I don't know exactly the words that I -- I'm not sure -- what's wrong with a primary situation?”

O’Malley, who is polling at just 2 percent in the latest Fox News poll, also hit Sanders on his changed party affiliation for the 2016 campaign.

With pride in his tenure as the “longest-serving Independent in the history of the United States Congress,” Sanders responded, saying, “I made a decision in this presidential election that I will run as a Democrat. I am a Democrat now.”

Sanders also spoke about his other Democratic primary opponent, Hillary Clinton. Sanders has shifted the tone of his campaign lately, focusing more on drawing differences between himself and the former secretary of state.

On Sunday, Sanders said he and Clinton "agree on a number of issues."

"On her worst day, Hillary Clinton will be an infinitely better candidate and president than the Republican candidate on his best day," Sanders said Sunday. "But having said that, we have very significant differences.”

ABC News' MaryAlice Parks contributed to this report.