WASHINGTON — The White House has berated Senate Republicans for writing to Iran’s leaders warning them against a nuclear agreement with President Obama, saying their letter skirts the Constitution and sends a dangerous and conflicting message.

In a lengthy and harshly worded statement released late Monday, Vice President Joseph R. Biden Jr., a Senate veteran of more than three decades and a former chairman of the Foreign Relations Committee, said he could recall no other instance in which senators had written to the leaders of another country, “much less a foreign adversary,” to say the president had no authority to strike a deal with them.

“This letter, in the guise of a constitutional lesson, ignores two centuries of precedent and threatens to undermine the ability of any future American president, whether Democrat or Republican, to negotiate with other nations on behalf of the United States,” Mr. Biden said. “Honorable people can disagree over policy. But this is no way to make America safer or stronger.”