Senate lawmakers on Thursday approved a proposal that would require the military's top commander in Afghanistan to testify before Congress -- but not until after President Barack Obama announces whether he will increase troops to the war-torn state.

The measure, sponsored by Senate Armed Forces Committee Chairman Carl Levin (D-Mich.), cleared the chamber on a 60-39 vote on Thursday afternoon. It supplants a similar effort proposed Wednesday by Sen. John McCain (R-Ariz.), who instead wanted McChrystal to testify before Obama announced his long-awaited troop decision.

"The fact is, during the campaign and as short as a month ago, the president said this was a war of necessity, and one we cannot afford to lose," McCain said on Wednesday. "We cannot afford to have the Taliban regain control of Afghanistan... I think it's pretty obvious we should have Gen. McChrystal and Gen. (David) Petraeus up here to testify."

Lawmakers' renewed concern about the war in Afghanistan hit its boiling point in September, after reporters discovered a memo from McChrystal to Obama predicting failure unless the United States deployed additional troops.

Both parties' lawmakers, however, seem diametrically opposed to that very suggestion. Many Democrats would prefer a strategy that focuses on counter-terrorism and relies less heavily on combat forces, while Republicans support McChrystal's anticipated recommendation for a new "surge" strategy in Afghanistan.

Of course, the two parties agreed that McChrystal should testify. But Thursday's vote to summon the general after the president makes his decision is likely to anger the GOP, which probably hoped to use McChrystal's testimony to pressure Obama on the issue.