WASHINGTON — Anger and outrage grew Tuesday over a letter from Senate Republicans to Iranian leaders designed to scuttle a yet-to-be-completed deal on its nuclear program.

Congressional Democrats and independents, even some who question a deal with Iran, called the letter authored by freshman Sen. Tom Cotton, R-Ark., an unprecedented act of political sabotage aimed at President Obama.

“I cannot imagine the Congress of the United States writing a letter to Khrushchev in the midst of those discussions and saying, ‘Don’t worry about this guy Kennedy, he doesn’t speak for our country,” said Sen. Angus King, a Maine independent, harkening back to the tense Cuban missile crisis showdown between President John F. Kennedy and Soviet Premier Nikita Khrushchev. “And yet that essentially is what took place (Monday).”

Iranian Foreign Minister Javad Zarif dismissed the letter as “mostly a propaganda ploy” designed to undermine the talks between his country, the United States and five other world powers.

Clearly annoyed by the lecturing tone of the letter, Zarif in turn lectured Cotton and his Senate colleagues.

“I should bring one important point to the attention of the authors and that is, the world is not the United States, and the conduct of inter-state relations is governed by international law, and not by U.S. domestic law,” Zarif said in a statement reported by Fars News Agency.

The 47 Republican signers held firm Tuesday, saying that offering their opinion of a nuclear deal in a message to Tehran’s leaders fulfilled part of their “advise and consent” role as senators.

“We have held for some time here in Congress, both Democrats and Republicans, that we ought to become more involved in foreign policy,” said Sen. Pat Roberts, R-Kan.

The support was not unanimous.

Maine’s Sen. Susan Collins, who was one of seven Senate Republicans who refused to sign the letter, expressed doubt about her colleagues’ tactic of skirting the White House and trying to affect foreign policy by going directly to Tehran.

“It’s more appropriate for members of the Senate to give advice to the president, to Secretary Kerry and to the negotiators,” Collins said in a statement. “I don’t think that the ayatollah (Iran’s supreme leader, Ayatollah Ali Khamenei) is going to be particularly convinced by a letter from members of the Senate.”

Sen. Bob Corker of Tennessee, chairman of the Senate Foreign Relations Committee, said Tuesday he “didn’t think it was going to further our efforts to get a place where Congress would play the appropriate role that it should in the Iran negotiations.”

In his letter, Cotton informed Iranian leaders that “Congress plays a significant role” in ratifying agreements and that anything not approved by Congress is “nothing more than an executive agreement between President Barack Obama and Ayatollah Khamenei.”

He warned that a nuclear deal probably wouldn’t survive beyond Obama’s presidency.

The Obama administration continued to vent its anger Tuesday. White House deputy press secretary Eric Schultz called the letter “reckless,” “irresponsible,” “misguided,” and “a flagrant, partisan attempt to interfere with the negotiations.”

A debate raged off Capitol Hill as well over the letter. The New York Daily News front page had “Traitors” in bold black letters with pictures of Senate Majority Leader Mitch McConnell, R-Ky., Paul, Cruz and Cotton. The hashtag #47traitors was among the most talked about topics on Twitter.

“I think the term ‘traitor’ is out of bounds,” said Sen. Roberts, a former Marine.

Some academics and bloggers accused the signers of violating the Logan Act, a 1799 law that says it’s a crime for any unauthorized person to “commence or carry on any verbal or written correspondence or intercourse with any foreign government” with the intention “to influence the measures or conduct” of the foreign government.