Republican 2016 presidential candidate and U.S. Sen. Marco Rubio sent a letter to Secretary of State John Kerry Monday vowing to work to block any U.S. ambassador nominations in Cuba unless the Obama administration meets four criteria on relations with the island nation beforehand. In the letter, Rubio wrote that unless Cuba is pushed to adopt political reforms, return terrorists living in Cuba to the U.S., resolve property rights issues and remove restrictions on American diplomats, he will do everything he can to block the nomination process in the U.S. Senate.

Rubio has long been a critic of President Barack Obama's policies on Cuba and most recently condemned the State Department's decision to remove Cuba from a list of state sponsors of terrorism last week. Rubio's parents arrived to the United States in the 1950s from Havana.

"By conditioning any normalization of relations with Cuba on these topics and other areas, the U.S. can leverage the prospect of improved bilateral relations to obtain tangible benefits for both the American and Cuban peoples," Rubio wrote. “It is important for the United States to continue being a beacon of freedom for the Cuban people."

Normalized relations with Cuba have become a frequent Republican criticism of the Obama administration since Obama announced talks between Washington, D.C., and Havana last year. A crowded GOP presidential field, including four U.S. senators, has resulted in candidates eager to draw a distinction between their proposed foreign policy doctrines and the Obama administration's diplomatic approach.

As a sitting senator, Rubio will be able to weigh in on the debate surrounding the appointment of an ambassador to Cuba. Any appropriations to fund opening an embassy in Cuba would also run through the Senate, a potential tool for Republicans to push back against U.S.-Cuba relations. U.S. Sen. Lindsey Graham in December promised to do everything in his power to stop funding for an embassy from going through. He announced Monday that he was also running for president.

Removing Cuba from the terror list was the next step in the direction of normalizing relations with the country that lies just south of Florida. Cuba had been on the list since 1982. An embargo against trade with Cuba remains in place and Congress appears unwilling to revise it.

According to FactCheck.org, the number of individuals with connections to terrorist groups in Cuba has been dwindling, and there is little evidence that Cuba provides training or weapons to terrorists.