Sen. Marco Rubio, R-Fla., a GOP primary rival to President-elect Trump, is emerging as one of his toughest early foreign policy critics in the Senate.

Rubio teamed up by Sen. Edward Markey, D-Mass, a fellow member of the Foreign Relations Committee, on Wednesday to urge Trump to strengthen the U.S. system of longstanding alliances, including NATO and the U.S.-Japan Mutual Security Treaty.