WASHINGTON  With his experience and leadership credentials under sharp criticism, Senator Barack Obama and his advisers are trying to clarify what has emerged as a central tenet of his proposed foreign policy: a willingness to meet leaders of enemy nations.

In an interview on Wednesday, Mr. Obama, of Illinois, sought to emphasize, as he and his aides have done continually over the last few days, the difference between avoiding preconditions for talks with nations like Iran and Syria, and granting them automatic discussions at the presidential level.

While Mr. Obama has said he would depart from the Bush administration policy of refusing to meet with certain nations unless they meet preconditions, he has also said he would reserve the right to choose which leaders he would meet, should he choose to meet with them at all.

The issue presents one of Mr. Obama’s biggest political and policy tests yet as he appears headed toward a general-election contest against Senator John McCain of Arizona: How to continue to add nuance to a policy argument that he views as a winning one, without playing into a fierce round of accusations that he is either shifting positions or appeasing the enemy.