WASHINGTON -- When President Barack Obama unveils his new Afghan war strategy Tuesday, he will face an immediate political fight over how to pay for it.

After a months-long review, Mr. Obama summoned Defense Secretary Robert Gates and other top military and civilian officials Sunday night to the White House to brief them on his decision to send tens of thousands of new U.S. troops to Afghanistan. The president will detail the plan in a prime-time speech Tuesday.

The troop announcement will set off a weeklong White House effort to persuade a skeptical Congress to back the new war strategy -- and to approve the tens of billions of dollars in new spending at a time of ballooning federal deficits.

It may be a tough sell. The hefty price tag of the pending Afghan troop increase is already drawing opposition from many Congressional Democrats, deepening Mr. Obama's estrangement from his own party over the conflict.

Some Democrats are coalescing around a new proposal to levy a war tax to help fund the conflict. The proposal by Wisconsin Democratic Rep. David Obey, who chairs the House Appropriations Committee, would impose a 1% tax on most Americans that rises to 5% for wealthier citizens. The administration has yet to weigh in on Mr. Obey's proposal, which would likely have a difficult time getting passed.

"If the president intends to go in over our objections, he should have to bear the burden of asking for a tax to pay for it," said Rep. Mike Honda (D., Calif.), a member of the House Appropriations Committee who supports the new tax. "You're talking about $30 billion or $40 billion per year in new spending. It's expensive."

Associated Press

Georgia National Guardsman Sgt. Scott A. Millican, right, and Air Force Technical Sgt. Phillip M. Huaser looks up as U.S. fighter jets fly overhead in Afghanistan Monday.

Congressional Republicans have pressed Mr. Obama to fully heed his commanders' requests for more troops and military resources. Republicans, who oppose the idea of a war tax, generally favor borrowing the additional money necessary for the Afghan surge or reallocating other government funds.

White House spokesman Robert Gibbs says Mr. Obama "will touch on cost" in his Tuesday speech, but won't go into specifics about how he wants to fund expansion of the war effort. "I don't think that the intention of the speech is to lay out a lengthy discourse on that," Mr. Gibbs said.

The administration's dispute with Congressional Democrats like Mr. Honda reflects stark statistics about the Afghan war's rising costs. The U.S. is currently spending roughly $3.6 billion a month in Afghanistan, a figure that will increase significantly once additional troops deploy there.

The White House estimates that sending 30,000 reinforcements to Afghanistan would add approximately $30 billion in new costs per year, or about $1 million per soldier or Marine. The Pentagon estimates that it will cost roughly $500,000 per soldier, for a total of about $15 billion per year in new expenses.

Independent budget analysts generally believe the Pentagon's numbers are more accurate, but acknowledge that it is hard to generate accurate estimates of how much the administration's decision to expand the Afghan war will ultimately cost.

"The arithmetic that's behind all of these numbers has never been made visible," said David Berteau, director of the Defense-Industrial Initiatives Group at the Center for Strategic and International Studies, a nonpartisan Washington think tank. "The astonishing thing is that there is no good summary of what we're getting for all of that money."

The Pentagon has already signaled that the U.S. has for the first time begun spending more money in Afghanistan than in Iraq. The administration's fiscal 2010 budget for the Pentagon, released in May, asked for $65 billion for Afghanistan and $61 billion for Iraq.

Defense officials say the administration will ask Congress to approve a separate Afghan war supplemental spending bill in coming months to fund the new troop deployments there.

Gen. James Conway, the commandant of the Marine Corps, said many of the Afghan war's high costs result from the difficulty and expense of trucking fuel, food and other supplies into the landlocked country.

In a speech in mid-October, Gen. Conway said military-grade fuel -- which costs roughly $1 a gallon in the U.S. -- can sometimes cost the Marine Corps about $400 per gallon once all the expenses of ferrying it into Afghanistan are factored in. The Marines operating in southern Afghanistan consume more than 88,000 gallons of the fuel per day, he said.

"Most all of that comes along this fairly tenuous supply line across Pakistan, where we're paying large amounts of money to tribes so that they don't fight each other and so that they don't raid our supply lines," Gen. Conway said at an energy conference in Virginia.

Marine Col. T. C. Moore recently visited Afghanistan at the helm of the Marine Energy Assessment Team, which Gen. Conway created to look for ways to reduce Marine energy expenditures. Col. Moore said most of the fuel costs were incurred once the gas arrived in Afghanistan and was pushed out to small Marine bases throughout the volatile area.

"The last tactical mile costs the most money, because it's simply so dangerous," he said.

Write to Yochi J. Dreazen at yochi.dreazen@wsj.com and August Cole at august.cole@dowjones.com