"Lieberman ought to be ashamed of himself for some of the things he said tonight, not as a Democrat but as an American," Robert Gibbs said on "Larry King Live" Tuesday night, CNN reported.

Lieberman is an independent senator from Connecticut who was the Democratic vice presidential running mate to Al Gore in 2000. This year Lieberman is supporting Sen. John McCain, R-Ariz., for president.

Tuesday night, Lieberman told delegates at the Republican National Convention Sen. Obama, D-Ill., has not reached across party lines to achieve anything significant in the U.S. Senate.

Gibbs called that "a lie," noting Obama worked with Sen. Richard Lugar, R-Ind., to keep nuclear weapons from terrorists and with Sen. Tom Coburn, R-Okla., on the federal budget.