WASHINGTON — President Obama on Tuesday nominated the first United States ambassador to Cuba in more than a half-century, defying opponents of his policy of rapprochement with the government of President Raúl Castro in an effort to further cement a new relationship between the countries before he leaves office.

Mr. Obama selected Jeffrey DeLaurentis, a career Foreign Service officer who has served since 2014 as chief of mission for the United States in Havana, to fill the post. He will have to be confirmed by the Senate, where Mr. Obama’s efforts to repair relations with Havana have been met with Republican resistance.

In a sharply worded statement issued shortly after Mr. Obama announced Mr. DeLaurentis’s selection, Senator Marco Rubio, Republican of Florida, said the nomination should not advance.

“Just like releasing all terrorists from Guantánamo and sending U.S. taxpayer dollars to the Iranian regime, rewarding the Castro government with a U.S. ambassador is another last-ditch legacy project for the president that needs to be stopped,” said Mr. Rubio, the son of Cuban immigrants. “This nomination should go nowhere until the Castro regime makes significant and irreversible progress in the areas of human rights and political freedom for the Cuban people, and until longstanding concerns about the Cuban regime’s theft of property and crimes against American citizens are addressed.”