"Joe Lieberman has said things that are totally irresponsible when it comes to Barack Obama. Here we have a leader for the future, really a great leader for the future and one that comes along only every now and then, and they know it so they have to undermine him. And one of their best weapons, of course, is someone who is considered by some to be a Democrat," Pelosi said on a San Francisco radio show.
Lieberman, the former Democratic vice presidential nominee, has been highly critical of Obama including a suggestion this week Obama had not always put his country first.
But despite the remarks, Pelosi said there is little Democrats can do, for now.
"The Democrats in the Senate are in a tough spot. They have 51 votes. Joe Lieberman organizes with them," Pelosi said.
"In 85 days or something, they will have five more Democrats they won't need him to make the majority. And it will be interesting to see what the leadership in the Senate, the Democratic leadership in the Senate, does at that point in terms of Joe Lieberman's chairmanship of his committee."