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Twenty-seven senators, Democrats as well as Republicans, sent Obama a letter last week pressing for a shift in Afghanistan strategy and major troop cuts.

“Given our successes, it is the right moment to initiate a sizable and sustained reduction in forces, with the goal of steadily redeploying all regular combat troops,” the senators wrote. “The costs of prolonging the war far outweigh the benefits.”

Arizona Sen. John McCain, the top Republican on the Senate Armed Services Committee, differed with that assessment. He told ABC’s “Good Morning America” on Tuesday that he agreed with Gates in hoping the withdrawal would be “modest.”

“I believe that one more fighting season and we can get this thing pretty well wrapped up,” McCain said.

There is broad public support for starting to withdraw U.S. troops. According to an Associated Press-GfK poll last month, 80 percent of Americans say they approve of Obama’s decision to begin withdrawal of combat troops in July and end U.S. combat operations in Afghanistan by 2014. Just 15 percent disapprove.

Obama has tripled the number of U.S. forces in Afghanistan since taking office, bringing the total there to about 100,000. The 30,000-troop surge he announced at the end of 2009 came with the condition that he would start bringing forces home in July 2011.

The president took months to settle on the surge strategy. This time around, aides say the process is far less formal and Obama is far more knowledgeable about the situation in Afghanistan than he was in 2009, his first year in office.

With the troop withdrawal set to begin next month, U.S. officials in Afghanistan said that military operations will become more focused and less ambitious over the coming three years. As troop levels decline the military will shift from a comprehensive counterinsurgency doctrine, which emphasizes small military-led development projects to gain the confidence of the local population, to counterterrorism, which focuses on capturing and killing insurgents, officials said.

The officials spoke on condition of anonymity because Obama has not announced the troop plan yet.

There are also indications that the administration, having learned from the U.S. experience in Iraq, will set deadline dates for the drawdown as it progresses, in order to keep pressure on the Afghans and give Congress mileposts.

With Iraq as a blueprint, commanders will need time to figure out what they call “battlefield geometry” — what types of troops are needed where. Those could include trainers, intelligence officers, special operations forces, various support units — from medical and construction to air transport — as well as combat troops.

Much of that will depend on where the Afghan security forces are able to take the lead, as well as the state of the insurgency. Part of the debate will also require commanders to determine the appropriate ratio of trainers versus combat troops.

Associated Press writer Lolita C. Baldor and Matthew Lee in Washington and Solomon Moore in Kabul, Afghanistan contributed to this report.