Bush's challenge isn't just to take control of Baghdad, but to win back control of his party.

From Newsweek:

A former senior Bush aide who is still close to the White House says if things don't improve, a delegation of Republican senators could one day show up in the Oval Office to tell Bush that the party is no longer with him and the war must end—much like Sen. William Fulbright's forcefully urging Lyndon Johnson to bring the Vietnam War to a close. Bush's challenge isn't just to take control of Baghdad, but to win back control of his party. "Before this, the president's credibility was hanging by a thread," says the former aide. "After this, I don't know. It may be lost."