"The presidency of the United States is not some crown to be passed between two families," Mr. O'Malley said on ABC's "This Week."

Until now, Mr. O'Malley's refusal to attack Mrs. Clinton outright has prompted detractors to suggest that he was running more for vice president than to be the party's eventual nominee.

On Sunday, Mr. O'Malley said repeatedly that "new perspectives and new leadership are needed."

And he said that Democrats needed to nominate someone who was willing to take on "wealthy special interest groups." Asked if Mrs. Clinton fit that description, Mr. O'Malley demurred.