Lawmakers from President Barack Obama's own Democratic Party expressed deep skepticism about the plan's chances for success but conceded they had little likelihood of blocking it. Republican supporters of the troop increase had their own objections, chiefly Obama's announcement Tuesday night of a July 2011 timeline for beginning to bring American forces home.
The stakes are great - al-Qaida's ability to regroup and plan the next terrorist attack on Americans - Obama's top advisers warned during a day of hearings before House and Senate panels.
"We cannot defeat al-Qaida and its toxic ideology without improving and stabilizing the security situation in Afghanistan," said Defense Secretary Robert Gates.
With voter support of the war on the decline, Democrats questioned the escalation and sought assurances that Obama's target date to begin withdrawing troops was firm.
"It seems to me that the large influx of U.S. combat troops will put more U.S. Marines on street corners in Afghan villages, with too few Afghan partners alongside them," said Sen. Carl Levin, D-Mich., chairman of the Senate Armed Services Committee.
The hearings were aimed at building support among war-weary lawmakers for Obama's dramatic rewrite of the battle plan in Afghanistan. By the end of next summer, the president plans to increase to 100,000 the number of U.S. troops there, marking the largest expansion of the war since it began eight years ago. The new strategy also relies on a pledge by NATO to commit an additional 5,000 to 7,000 troops.
Rep. John Murtha of Pennsylvania, a vocal war critic who is a senior House Democrat overseeing military spending, predicted that Congress would pass a $40 billion war financing bill early next year to pay for the added deployments.
Murtha said he remains unconvinced the troop increase is a good idea but believes he and other anti-war Democrats will not be able to stop it. "It's not likely that there would be any circumstances where the president would lose this battle this year," he said.
At the same time, Murtha said he didn't think more troops would make the U.S. more secure.
"Al-Qaida can go any place. They don't have to be in Afghanistan," he told reporters.
Despite the war's waning popularity among voters, there were few protesters on hand at Wednesday's hearings.
Republicans objected to the setting of a hard deadline for withdrawing troops and said Obama must be willing to delay the start of a pullout if security deteriorates.
"We don't want to sound an uncertain trumpet to our friends in the region," said John McCain, the Senate panel's top Republican and Obama's opponent in last year's presidential race.
As part of a full-court press by the White House to make the case for Obama's new strategy, Gates, Secretary of State Hillary Rodham Clinton and Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, argued for the troop increase. But they also were careful with their words so as not to aggravate divisions on the issue. They cast the war as serious but not hopeless. Mullen said the Taliban had regained ground in Afghanistan - gaining "dominant influence" in 11 of 34 provinces - but could be defeated with enough resources and time.
"While there are no guarantees in war, I expect that we will make significant headway in the next 18-24 months," he said. Gates told lawmakers that the situation is far less dire than the violent chaos that gripped Iraq in 2006. Still, he said, "This will take more patience, perseverance and sacrifice by the United States and our allies."
He called the region the "epicenter of extremist jihadism," reminding lawmakers that local and foreign Muslims had joined forces there before - in defeating the former Soviet Union. "For them to be seen to defeat the sole remaining superpower in the same place would have severe consequences for the United States and the world," Gates said.
He suggested the July 2011 withdrawal date was both firm and flexible, frustrating lawmakers who said that wasn't possible. "Which is it? It's got to be one or the other," said McCain.
When pressed, Gates said the beginning of drawing down troops would not necessarily be based on conditions in Afghanistan and that the president was committed to begin pulling at least some troops out by the target date
At the same time, the president will have the authority to change gears after the Defense Department conducts a formal assessment in December 2010. Gates said the July 2011 date was chosen because it would give the Marines two years to complete a security push in Helmand province that began last July.
"I do not believe we have locked ourselves into leaving," added Clinton. "But what we have done ... is to signal very clearly to all audiences that the United States is not interested in occupying Afghanistan."
She added that Afghan President Hamid Karzai's pledge to fight corruption "must now be met with action."
The buildup also will put more strain on troops by giving them less time than hoped for at home.
Mullen said supplying the extra forces for Afghanistan while there are still so many troops in Iraq will mean putting off for a couple of years the goal of lengthening the time they rest and retrain at home between tours of duty - a period the military calls "dwell time." The Army had been moving toward giving two years of dwell time between each one-year tour.
After meeting Wednesday with Karzai, U.S. Gen. Stanley McChrystal called Karzai's reaction to the new U.S. strategy "really positive. The president was very upbeat, very resolute this morning."
NATO Secretary-General Anders Fogh Rasmussen said he expected the allies to bolster the American buildup with more than 5,000 additional troops. Gates said the allies would remain focused on the less volatile north and west of Afghanistan to "prevent the insurgency from establishing new footholds," while Americans focus on the south and east.
French President Nicolas Sarkozy hailed Obama's speech as "courageous, determined and lucid" but stopped short of pledging additional French troops.
Associated Press writers Slobodan Lekic in Brussels, Heidi Vogt in Kabul and Stephen Ohlemacher in Washington contributed to this report.