WASHINGTON (AFP) -- Democrats and Republicans slammed a new U.S. plan to surge troops into Iraq as a tragedy and the worst blunder since Vietnam, as the Bush administration affirmed it had no idea how long the forces would stay.

President George W. Bush and top aides faced fierce criticism as they launched a hard sell to convince a nation weary of a war which has claimed 3,000 U.S. lives that the last-ditch bid to step up the battle can work.

They warned the government of Iraqi Prime Minister Nuri al-Maliki was on "borrowed time" and promised to hold his government to its end of the bargain to tackle sectarian violence before U.S. troops pour into Baghdad.

Secretary of State Condoleezza Rice faced a fearsome grilling by the Senate Foreign Relations Committee over the plan to send 21,500 more troops to Iraq.

"I think this speech given last night by this president represents the most dangerous foreign policy blunder since Vietnam, if it is carried out," said Republican Senator Chuck Hagel.

Senator Joseph Biden, who presided over the committee's first inquisition of a senior administration figure since Democrats took the reins of Congress last week, said Americans' prayers for a workable strategy had not been met.

"I believe the president's strategy is not a solution, Secretary Rice. I believe it's a tragic mistake," Biden said. "The result will be the loss of more American lives and our military stretched to the breaking point with little prospect of success."

Democrats said they would hold votes on the Bush plan, but stopped short of vowing to cut off war funding.

The president, whose political legacy is riding on the long-odds bid to pacify Iraq, appeared with U.S. troops in Fort Benning, Georgia, with his own personal sales pitch on Thursday. "The new strategy is not going to yield immediate results, it's going to take a while," he told the troops. "Yet, over time, we can expect to see positive results."

Rice meanwhile defended the Bush administration's reliance on the much-derided Maliki regime, which has failed to meet past U.S. demands for robust action.

"I think he knows that his government is, in a sense, on borrowed time," she said.

Defense Secretary Robert Gates said no one could predict how long the U.S. troop surge in Iraq would last.

"It's viewed as a temporary surge, but I think no one has a really clear idea of how long that might be," Gates said, adding the United States would closely monitor Iraqi compliance before the first new troops arrived in Iraq.

Democrat Christopher Dodd, a 2008 presidential candidate, described the U.S. Iraq policy as a "fool's paradise." Even staunch Republican Senator Norm Coleman said "the cost is too great" in terms of U.S. lives to support the Bush plan.

Bush said in Wednesday's speech his new push was aimed at crushing terrorists, insurgents and rogue militias, and helping Iraq's security forces take control of the entire country by November.

In a rare specific admission of failure, Bush pointed to past efforts to secure Baghdad saying that not enough U.S. and Iraqi troops had been deployed.