WASHINGTON (AP) - President Barack Obama is preparing to announce a sweeping overhaul of the U.S. strategy in Afghanistan on Tuesday, including the addition of thousands more American forces, a clarification of the mission and a path toward disengagement. It will likely be one of the toughest sales jobs of his presidency.

Democrats in Congress already are setting tough conditions - if not outright opposition to a deeper U.S. involvement - and the American public is increasingly negative about the 8-year-old conflict that has become a serious drain on U.S. resources in a deeply troubled economic period. Casualties have increased sharply and are likely to grow more with the addition of more troops.

Congressional uneasiness or opposition was voiced Sunday by the leading Senate Democrat on military matters, who said any plan to significantly expand U.S. troop levels must show how those reinforcements will help increase the number of Afghan security forces.

Remarks by Sen. Carl Levin of Michigan, chairman of the Senate Armed Services Committee, were a preview of the possible roadblocks as the president begins to sell a broader, more expensive battle plan for Afghanistan to an American public weary of the conflict.

Greater numbers of Afghan army and police are central to succeeding in the war, according to Levin, and more U.S. trainers and an infusion of battlefield gear will help meet that goal. But Levin said that it's not clear what role the tens of thousands of additional U.S. combat troops would play in that buildup, and he said Obama has to make a compelling case for it during the nationally broadcast 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," Levin said. "We cannot, by ourselves, win (the) war."

Another facet of Obama's plan appears to be an expanded partnership with Pakistan as part of U.S. pressure on that country's shaky government to do more to root out extremists based along Pakistan's borders with Afghanistan.

The Washington Post reported Monday that Obama had sent a letter to Pakistan President Asif Ali Zardari saying the U.S. planned no early withdrawal from Afghanistan and will increase its military and economic cooperation with Pakistan. The Post, quoting unidentified administration officials, also said that Obama called for closer collaboration against extremist groups, including five named in the letter.

The letter, delivered by national security adviser James Jones, reportedly included a blunt warning that the U.S. would not tolerate support within Pakistan's military and intelligence operations of extremists fighting in Afghanistan.

At West Point, Obama was 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.