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.

"For more than 70 years, the United States has helped to construct and maintain the institutions of global peace," the senators wrote to Trump in a letter. "Among the most important of these institutions are defensive alliances such as NATO, as well as multilateral treaties comprising the nonproliferation regime."

"We urge you to affirm that the United States will continue to support these institutions," Rubio and Markey wrote.

Just Tuesday, Rubio also issued the harshest statement about Trump's decision to nominate Exxon Mobil CEO Rex Tillerson for the position of secretary of state, saying he has "serious concerns about the nomination" and wants more information about Tillerson's ties to Russia and its president, Vladimir Putin.

During the presidential campaign, Trump often questioned the value of the NATO alliance. Trump argued that the organization needs to be dramatically overhauled and suggested that the U.S. may not come to the aid of NATO countries in a time of crisis or in the face of Russian aggression if they didn't pay their fair share of their dues and provide the military commitment expected of them.

Referring to these campaign comments, Rubio and Markey said it is important "U.S. defense guarantees be clear and unequivocal."

"While we agree that it is essential for our allies to spend more on their defense, U.S. security guarantees should not be conditioned on additional monetary compensation," they wrote.

Japan and South Korea, they argued, also depend on U.S. guarantees to deter threats to their security in exchange for giving up any pursuit of their own nuclear arsenals.

"Weakening our security guarantees could prompt these and other countries to reconsider their nonproliferation commitments," they added.

On the stump, Trump set off a firestorm at home and abroad by suggesting that more countries could become nuclear powers without damaging the United States' security interests.

Rubio and Markey warned just the opposite – that if U.S. allies attain nuclear weapons it would "exacerbate the risk of accidental nuclear conflict and increase pressure for preventive attacks by adversaries."