TEHRAN — Iran’s highest leader issued a sharp response Thursday to a letter to the country’s leadership by Republican lawmakers, deriding it as an indication that Washington is “disintegrating” from within.

Ayatollah Ali Khamenei, Iran’s supreme leader, said the letter warning that any nuclear deal could be scrapped by a new president was “a sign of a decline in political ethics and the destruction of the American establishment from within.” The statement was posted on his website.

Mr. Khamenei, who will have the final say in Iran over a nuclear deal, characterized the open letter written by 47 Republican senators on Monday as a reflection of Washington’s decadence.

“All countries, according to the international norms, remain faithful to their commitments even after their governments change, but the American senators are officially announcing that at the end of the term of their current government, their commitments will be considered null and void,” Mr. Khamenei wrote. He said the letter was a “sign of declining political ethics.”