Mr. Romney’s split with Mr. Reiss was more public. A former head of policy planning at the State Department, Mr. Reiss argues that it makes sense for the United States to negotiate with the Taliban as a way to end the war in Afghanistan.

“There are always risks in talking with any terrorist group, and the Taliban are no different in this respect,” Mr. Reiss wrote in a blog post on the Web site of Foreign Policy magazine. But he added, “Some type of a negotiated solution is the best near-term bet to halt the fighting.”

During the Republican debate on Monday, Mr. Romney was asked by Bret Baier of Fox News whether his adviser was wrong.

“Yes,” he replied. “The right course for America is not to negotiate with the Taliban while the Taliban are killing our soldiers. The right course is to recognize they’re the enemy of the United States.”