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But Chris Preble, of the libertarian-leaning Cato Institute, said Mr. Rubio inherited the neoconservative approach from Sens. John McCain of Arizona and Lindsey Graham of South Carolina, military hawks who supported the wars in Iraq and Afghanistan, as well as U.S. involvement in Libya and have more recently urged lawmakers to arm Syria’s rebels — a position Mr. Rubio has not taken.

“It is an interventionist point of view, which generally is inclined to have the U.S. military involved and U.S. government involved in foreign conflicts, with the idea that our being involved will allow us to shape it in our interests,” Mr. Preble said.

Mr. Carafano said both been are learning on the fly.

“My guess is they are going to migrate more toward the center,” Mr. Carafano said. “I think they are both on a journey, and they are both growing and maturing.”