The former Republican, who was President Reagan's Navy secretary, said in the Democrats' weekly radio address that Bush's "incompetence" in Iraq had undercut the fight against terrorism.
Webb is locked in a close race in Virginia against Republican Sen. George Allen that could determine whether the Senate remains in GOP control.
"Since 2003, President Bush has laid out nine different plans for victory in Iraq, none of them serious and none of them workable. And most seriously, this incompetence has hindered our ability to fight international terror," Webb said.
It marked the second time since July 1 that Webb, a decorated Vietnam combat veteran, has given the Democrats' address. Both times, his focus has been Iraq.
Webb warned in a newspaper column in 2002, the year before Bush ordered the Iraq invasion, that a war there would destabilize the oil-rich Middle East and mire U.S. forces in a bloody and protracted conflict. As of Friday, 2,810 American troops had died in Iraq.
"It gives me no great pleasure today to be saying `I told you so,'" said Webb, whose son, Jimmy, is a Marine on active duty in Iraq. "It pains me as an American that our casualties are again escalating while this president and his followers are still incapable of bringing forward an intelligent, commonsense approach to ending our involvement there."
Webb cited Iraq and other Bush-backed policies among his reasons for leaving the GOP. Now, other Republicans are reaching the same conclusions he did about the war.
"Over the past several weeks a few realists in the Republican Party, such as (Virginia) Sen. John Warner and former Secretary of State Jim Baker, have begun to make their voices heard. They are moving away from the fantasy world of this administration, toward real solutions," Webb said.
Allen has been one of Bush's most reliable supporters of the war, but Allen began playing down his stance three weeks ago after Warner, the respected Armed Services Committee chairman, returned from Iraq with a grim assessment of increasing sectarian carnage there.
With polls nationally and in Virginia showing low popularity for both the president and the war, Allen sought to align himself with Warner on the issue. However, Allen has refused to publicly differ from Bush's intent to keep troops in Iraq through his term.
"A Democratic Congress will demand from day one that the president find a real way forward in Iraq. We'll work with the administration and other Republicans to develop a concrete plan, but none of us are ready to settle for empty rhetoric, or the same old unacceptable results," Webb said.
In the Saturday address, Webb did not mention the latest controversy in the Virginia Senate campaign. On Friday, Allen's campaign selected sexually explicit passages from Webb's six war novels and thrust them into the race, claiming they are demeaning to women.
Webb's fiction evokes events he witnessed as a Marine in some of Vietnam's bloodiest battles and the scarring effect they had on those who fought in them. It includes descriptions of sex and rape.