Washington ? The leading Senate Democrat on military matters said Sunday that President Barack Obama’s anticipated plan for significantly expanding U.S. troop levels in Afghanistan must show how those reinforcements will help increase the size of the Afghan security forces.

Sen. Carl Levin, chairman of the Senate Armed Services Committee, said more Afghan army and police are central to succeeding in the 8-year-old war and more U.S. trainers and equipment can help meet that goal. But it’s unclear, Levin said, what role tens of thousands additional combat troops will play, and Obama has to make a compelling case during a national address he’s scheduled to give Tuesday night from the U.S. Military Academy at West Point, N.Y.

“The key here is an Afghan surge, not an American surge,” said Levin, D-Mich. “We cannot, by ourselves, win (the) war.”

Levin’s remarks are a preview of the possible roadblocks Obama faces from his own party as he prepares to sell a broader, more expensive battle plan for Afghanistan to an American public weary of the conflict that began just weeks after the Sept. 11, 2001, attacks.

At West Point, Obama is expected to announce an increase of up to 35,000 more U.S. forces to defeat the Taliban-led insurgency and stabilize a weak Afghan government. The escalation, which would take place over the next year, would put more than 100,000 American troops in Afghanistan at an annual cost of about $75 billion. Obama is also expected to outline his exit strategy.

Democrats concerned over the price tag have proposed a war tax to pay for operations. Rep. David Obey, D-Wis., chairman of the House Appropriations Committee, has introduced legislation to impose a war surtax beginning in 2011. The bill would exempt service members and their families.

Lawmakers want a greater commitment from NATO allies so the U.S. isn’t footing the bill on its own.

“I’ve got a real problem about expanding this war where the rest of the world is sitting around and saying, ‘Isn’t it a nice thing that the taxpayers of the United States and the U.S. military are doing the work that the rest of the world should be doing?”‘ said Sen. Bernie Sanders, a Vermont independent.

British Prime Minister Gordon Brown has said several allied nations will offer a total of 5,000 more troops. But speaking Saturday at a news conference in the Caribbean nation of Trinidad, Brown also said Afghan President Hamid Karzai’s government must meet specific benchmarks that allow foreign troops to gradually hand over control of the fighting to local forces.

Sen. Richard Lugar of Indiana, the top Republican on the Foreign Relations Committee, said he was wary of strict benchmarks that put both sides in an untenable situation if they’re not met.