The White House's consideration of leaving no American troops in country after the 2014 pull out, known as the "zero option" is already having "a very significant damaging effect in Afghanistan," he told reporters on Capitol Hill.
"You never point a loaded weapon at somebody unless you are ready to pull the trigger," McCain said of the Pentagon's flip-flopping on the viability of a zero option in Afghanistan.
President Obama put the zero option back on the table for possible U.S. postwar plans after a contentious discussion with Afghan President Hamid Karzai over his decision to suspend talks for a U.S.-Afghan bilateral security agreement.
A security agreement is the linchpin to locking plans for a postwar U.S. force in Afghanistan after next year.
But McCain's comments come days after Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey said he has not been asked to prepare for a zero option.
But the four-star general warned that if Washington and Kabul cannot come to terms on a postwar deal for Afghanistan, then the only option will be a complete pullout of U.S. forces next year.
“No one asked me to prepare a zero option. I don’t recommend a zero option, but there could be a zero outcome, because we can only stay here if we are invited to do so,” Dempsey told reporters in Kabul on Monday.
Senate Armed Services Committee chief Sen. Carl Levin backed Dempsey's comments, adding the four-star general's assessment of postwar planning in Afghanistan was not off base.
Without a security agreement with Afghanistan, a zero option for U.S. troops there is "a fact of life."
"I am for keeping troops there," Levin added, but said his support for an American postwar force was dependent on locking in a bilateral security agreement with Kabul.
To back a post-2014 force in the country without a security deal "is not being honest" regarding the realities facing American military planners as the over decade-long war comes to a close.
That said, Dempsey's comments reflected the general's "best military advice" on the issue, but the Pentagon and White House still have "a large decision space" on postwar planning in Afghanistan, Pentagon press secretary George Little said Tuesday.
There remains "an ongoing discussion" on what the U.S. postwar force will be in Afghanistan or whether there will be any American troops on the ground in the country after 2014, Little told reporters at the Pentagon.
"The secretary is clear, all options are on the table [in Afghanistan]," Little said, noting there was still a year and-a-half before the final U.S. withdrawal.
The first 34,000 U.S. troops in Afghanistan are slated to return home this spring. The remaining 32,000 American forces will rotate stateside after the Afghan presidential elections in April 2014, ending the U.S. war in the country.
That said, the so-called "zero option" for Afghanistan remains on the list of potential outcomes being considered by Defense Secretary Chuck Hagel and administration officials for postwar Afghanistan.
"If the plan is for zero [troops] that's a relatively easy plan to develop," Little added.