Now that Mr. Romney has emerged as the likely Republican nominee and Afghanistan is again being tested by a Taliban offensive, his position on the war is likely to come under more scrutiny after a primary fight that gave him few opportunities to offer nuanced national security positions. Even so, analysts say he has reasons to be less than precise on Afghanistan: The war’s declining support among voters means there is little space for him to stake out a policy that provides both a sharp political contrast with Mr. Obama and keeps the war’s unpopularity at a distance.

“He doesn’t want to own this war in the event he gets elected, but by the same token he can’t look like he’s advocating a precipitous withdrawal for all sorts of reasons, including alienating the Republican base, and yet he cannot take the same position as the president,” said Stephen Biddle, a military expert at the Council on Foreign Relations. “It’s difficult to square the circle and meet all those constraints at the same time.”

And domestic politics are only one tricky element. There are serious doubts that the broadly hoped-for exit strategy of both parties — that Afghan forces can progress to where they can keep the Taliban at bay with limited assistance by 2014 — will materialize that quickly, if at all.