Sen. Russ Feingold (D-Wis.) plans to introduce legislation calling for a "flexible timetable" for withdrawing U.S. troops from Afghanistan.

Feingold, a critic of President Barack ObamaBarack Hussein ObamaThe Memo: Democrats confront prospect of long primary George Conway: 'If Barack Obama had done this' Republicans would be 'out for blood' George Conway to take part in MSNBC impeachment hearing coverage MORE's decision to escalate troop levels in Afghanistan, plans a bill that would seek to define what he's called the United States's "large-scale, open-ended military strategy" in the region.

The new legislation comes as tensions between the U.S. and Afghan leaders have become more publicly apparent over the past week, with some U.S. officials openly expressing frustration toward Afghan President Hamid Karzai over the corruption that's plagued his government.

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White House Press Secretary Robert Gibbs declined Tuesday to say whether or not the administration considers Karzai an "ally," while former U.N. envoy to Afghanistan Peter Galbraith suggested that Karzai has been using drugs and is emotionally unstable.

It's not clear as of yet how Feingold's bill would be structured. The liberal magazine The Nation reports that his legislation would be introduced as soon as next week, along with a companion bill by Rep. James McGovern (D-Mass.) in the House.

When Obama announced the troop surge for Afghanistan in December of last year, he spoke of beginning a withdrawal of troops from Afghanistan in July 2011, though administration officials subsequently sent signals that such a deadline may be softened depending on the situation on the ground. U.S. forces have been involved in Afghanistan since fall 2001.

Feingold reacted to comments by Karzai last week distancing himself from the U.S. by calling for more "achievable" counterterrorism goals in Afghanistan.

"Rather than pursuing a large-scale, open-ended military strategy in Afghanistan, we should focus on achievable counterterrorism goals," the Wisconsin Democrat said. "This approach would strengthen our national security by allowing us to focus on the global fight against al Qaeda.”