WASHINGTON — Days before President Obama is expected to announce his decision to send 30,000 or more additional U. S. troops to Afghanistan, key lawmakers from both parties expressed deep misgivings about the cost and course of an expanded war.

Skepticism from the president’s own party, along with new doubts raised by Republicans who generally have supported broadening the conflict, underscore the stakes for Obama as he prepares to unveil his troop increase proposal during a speech Tuesday at the U. S. military academy at West Point.

The critical comments from lawmakers came amid the release of a new report by the Senate Foreign Relations Committee that suggested the debate over sending more soldiers to Afghanistan might have been avoided if the United States hadn’t missed an opportunity to capture or kill al-Qaida leader Osama bin Laden at Tora Bora in 2001.

Sen. Richard Lugar, R-Ind., the ranking Republican on the Foreign Relations Committee, appeared to embrace a proposal gaining momentum among some Democrats for a war “surtax” to help defray the costs of expanding the Afghanistan campaign.

“I think we will have to pay for it,” Lugar said on CNN. “We may wish to discuss higher taxes to pay for it.”

Lugar also said he believes that Americans, already faltering in their support of the war, would not be willing to sustain the military campaign beyond five more years.

Similar questions about the war plan were raised Sunday by Sen. Carl Levin, D-Mich., chairman of the Senate Armed Services Committee, and Sen. Jack Reed, D-R. I.,a military veteran who serves on the Senate Armed Services and Appropriations panels.

“What we have to have is a continually decreasing military presence in Afghanistan,” Reed said, also on CNN. “Unless we’re on a trajectory in which our troop levels come down, the ability of the American public to support it and financially to support it is questionable.”

The cost of the war has become a central concern on Capitol Hill, where members already nervous about voter frustration with the nation’s economy are locked in an intense debate over an $850-billion health care overhaul bill.

The war in Afghanistan has cost $243 billion since 2001, and government calculations estimate the cost would raise $1 million per year for every additional U. S. soldier. At that rate, the increase Obama is expected to endorse could wipe out savings from troops withdrawn from Iraq.

Given the economic climate, Sen. Bernie Sanders, an independent from Vermont, said it would be “immoral” to escalate the war in Afghanistan without introducing new taxes or taking other measures to pay for it.

“No one is talking about bringing the troops home tomorrow,” Sanders said on ABC’s “This Week.” “But if you’re going to have a presence there, you just can’t pass the bill on, as we did in Iraq, to our kids and our grandchildren. I think that’s wrong.”

A decision to send an extra 30,000 U. S. troops would fall short of the 40,000 requested by Gen. Stanley McChrystal, the U. S. commander in Afghanistan. The Obama administration may seek to bridge that gap by leaning on European and other allies to expand their troop commitments. But prospects for success in doing so are uncertain at best.

Because of skepticism among Democrats, Obama probably will depend on support from Republicans for any expansion of the war. Sen. Lindsey Graham, R-S. C., also a member of the Armed Services Committee, said prevailing in Afghanistan should be the nation’s top priority, and he proposed trimming the health care bill to pay for it.

Afghanistan “is not just any place on the planet,” Graham said, also on ABC. “This is the place where the Taliban took control after the Russians left, aligned themselves with al-Qaida and attacked this nation and killed 3,000 Americans.”

The U. S. invaded Afghanistan shortly after the Sept. 11, 2001, attacks, ousting the Taliban and prompting al-Qaida fighters to flee into the mountainous region along the border with Afghanistan.

A new report from the Senate Foreign Relations Committee offers the most definitive account to date of bin Laden’s escape into Pakistan and concludes the United States allowed him to slip from its grasp in mid-December 2001.

Signal intercepts and other evidence confirm that bin Laden was holed up along with other al-Qaida fighters in the caves of Tora Bora, according to the report, based on a review of military histories as well as interviews with CIA and U. S. Special Operations officers involved in the battle.

Requests to send U. S. troops to seal the border were rejected by Gen. Tommy Franks, who was then leader of U. S. Central Command, as well as then-Defense Secretary Donald Rumsfeld. Instead, fewer than 100 U. S. special operations troops were involved in the pursuit, working with Afghan troops who were ill-equipped for the job.

Bin Laden eluded an intense bombing campaign and relied on a cease-fire ruse to slip across the border into Pakistan, where he is believed to be hiding still, according to the report.

“The failure to finish the job represents a lost opportunity that forever altered the course of the conflict in Afghanistan and the future of international terrorism,” the report said. Allowing bin Laden to escape was a major factor in “laying the foundation for today’s protracted Afghan insurgency and inflaming the internal strife now endangering Pakistan.”