Gephardt said Dean, the former governor of Vermont, has changed positions on issues like the North American Free Trade Agreement, Medicare cuts and a ban on assault weapons.
"It's become nearly impossible to know what Howard Dean really believes," the New York Times quoted Gephardt as saying.
"The only way we can beat George Bush is to be clear about where we stand and to be truthful with the American people, to me, there is no room for the cynical politics of manufactured anger and false conviction. I believe in standing for something, and I think all of you do, too."
Pundits say Gephardt needs a victory in the Iowa caucuses on Monday to keep his campaign alive.