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House Speaker John A. Boehner, Ohio Republican, said he hoped that Mr. Obama would continue to “listen to the commanders on the ground.”

Sen. John McCain, Arizona Republican, said the president’s plan “poses an unnecessary risk to the hard-won gains that our troops have made thus far.”

“This is not the ‘modest’ withdrawal that I and others had hoped for and advocated,” Mr. McCain said.

The administration is under growing pressure from lawmakers of both parties to scale down the 10-year-old war in the country that provided a haven for the plotters of the Sept. 11 attacks. More than 1,600 U.S. troops have been killed, and it’s costing taxpayers about $10 billion per month.

Some in Congress now call the nation’s debt crisis a bigger security threat than Al Qaeda.

House Minority Leader Nancy Pelosi, California Democrat, said many lawmakers hoped the withdrawal “would happen sooner than the president laid out — and we will continue to press for a better outcome.”

Mr. Obama also set forth a framework for a political settlement in Afghanistan as the troop withdrawal proceeds, including talks with Taliban leaders. “Our position on these talks is clear,” he said. “They must be led by the Afghan government, and those who want to be a part of a peaceful Afghanistan must break from Al Qaeda, abandon violence and abide by the Afghan Constitution.”

The surge has enabled NATO forces to make significant gains in Helmand and Kandahar provinces, which were Taliban strongholds. Lawmakers said Mr. Gates and Gen. David H. Petraeus, the top commander in Afghanistan, were pushing for an initial drawdown of 3,000 to 5,000 troops this year. There are now about 100,000 troops there.

Senior administration officials, speaking on the condition of anonymity prior to the president’s speech, said Afghanistan has not posed a threat of a terrorist attack against the United States for at least seven years. They said the primary threat is from al Qaeda operatives in Pakistan, where a Navy SEAL team killed Osama bin Laden about seven weeks ago.

Mr. Obama said the U.S. “will continue to press Pakistan to expand its participation in securing a more peaceful future for this war-torn region.” But the president didn’t provide specifics on how he intends to persuade Pakistan.

“This [drawdown] is not going to increase the threat,” an administration official said. “And it’s not going to affect at all the threat in Pakistan, either. Right now, the al Qaeda threat does come from Pakistan. That is where they are hunkered down.”

As for Chicago landing two conferences, much of the credit goes to Mayor Rahm Emanuel, Mr. Obama’s former chief of staff. A senior administration official said the White House considered other U.S. cities, but he acknowledged, “It wasn’t a very wide net that was cast.”