Senator Arlen Specter (Pennsylvania), a member of Bush's own Republican Party, was responding to Bush's recent statement that he was the "decision maker" on deployments to Iraq.
"I would suggest respectfully to the [U.S.] president that he is not the sole decider [on U.S. troop deployments to Iraq] -- that the 'decider' is a shared and joint responsibility [with Congress]," Specter said.
Specter's remarks came during a Senate hearing on the war powers of the U.S. Congress.
Meanwhile, former Secretary of State James Baker, a co-chairman of the Iraq Study Group, told a congressional hearing on January 30 that the White House should find a way to negotiate with Congress on troop additions.
He also said "there is no action the American military can take that, by itself, can bring success in Iraq."
Various draft congressional resolutions have been sharply critical of Bush's plan to send 21,500 additional troops to Iraq.
Earlier, Admiral William Fallon, the man nominated by Bush to head U.S. forces in the Middle East, said time was running out to improve security and promote political reconciliation in Iraq.
"What we've been doing is not working," Fallon told his Senate confirmation hearing on January 30.
(compiled from agency reports)