"The American people are going to want to know why we're here, they're going to want to know what our interests are, and the president is going to walk through the decision-making process," White House press secretary Robert Gibbs told reporters.

"We're in the ninth year of our efforts in Afghanistan," Gibbs said yesterday. "The president does not see this as an open-ended engagement."

Obama is set to speak at 8 p.m. Philadelphia time.

After months of review, Obama held his final session with top foreign-policy advisers and military commanders this week, including Gen. Stanley McChrystal, the U.S. commander in Afghanistan. The talks now focus on adding 30,000 to 35,000 U.S. troops, an official said.

McChrystal cautioned in August that the situation in Afghanistan, where a Taliban insurgency has gained strength, was "serious" and required a revised strategy.

To build support, Defense Secretary Robert M. Gates, Secretary of State Hillary Rodham Clinton, and Adm. Michael Mullen, chairman of the Joint Chiefs of Staff, will be dispatched to outline the strategy to Congress next week. The House Foreign Affairs Committee announced that Clinton, Gates, and Mullen would testify before it Wednesday.

McChrystal may testify the following week.

Obama is under pressure from both parties. Some Democrats are resisting deeper U.S. involvement in the war, while Republicans are pushing for quick action to add more troops to the fight.

The president also faces a skeptical public. In a recent Washington Post-ABC News poll, 52 percent said the conflict was not worth the cost.

"It's a million dollars a troop for a year," Gibbs said. "Ten thousand troops is $10 billion. That's in addition to what we already spend in Afghanistan and Pakistan."

Michigan Democrat Carl Levin, chairman of the Senate Armed Services Committee, said last week that higher-income Americans should be taxed to pay for sending more troops to Afghanistan.

An "additional income tax to the upper brackets, folks earning more than $200,000 or $250,000" a year, could fund more troops, Levin, a Michigan Democrat, said in an interview for Bloomberg Television's Political Capital With Al Hunt.

Obama has given no indication of how many U.S. military personnel he will need to achieve his goals, which include dismantling al-Qaeda's networks in the region and preparing the Afghans to take responsibility for security.

The United States now contributes 68,000 of the 110,000 international troops in Afghanistan. McChrystal has recommended boosting the U.S. force by 40,000.