WASHINGTON (AP) — Congressional Democrats on Tuesday accused Senate Republicans who signed a letter to Iran’s leadership of undermining President Barack Obama in international talks aimed at curbing Tehran’s nuclear program and preventing the need for future military conflict.

In remarks on the Senate floor, Sen. Dick Durbin, D-Ill., pronounced the letter reckless, much as it would have been for U.S. lawmakers to “reach out to the Vietnamese” a generation ago.

He said he hoped it would not cause the negotiations to fail, adding that an attempt to avoid a nuclear-armed Iran “is something that should not be undermined for political ambition.”

Durbin spoke a day after nearly four dozen Republican senators sent their letter, a step that Obama and Vice President Joseph Biden both strongly condemned and White House spokesman Josh Earnest said reflected a “rush to war, or at least the rush to the military option.”

The letter’s lead author, Sen. Tom Cotton, R-Ark., denied undermining Obama’s negotiating position. Appearing on MSNBC, he said, “We’re making sure that Iran’s leaders understand that if Congress doesn’t approve a deal, Congress won’t accept a deal.”

He accused Iran of seeking “a nuclear umbrella so they can continue to export terrorism around the world.”

In an open letter Monday to the leaders of Iran, Republican lawmakers warned that unless Congress approved it, any nuclear deal they cut with Obama could expire the day he walks out of the Oval Office. It was signed by 47 of the Senate’s 54 Republicans, including members of the leadership and potential presidential candidates.

In a statement issued late Monday night, Biden said Republicans had “ignored two centuries of precedent” and he said the move “threatens to undermine the ability” of any future president to negotiate with foreign countries.

Biden, in his statement, noted that presidents of both political parties have negotiated historic international agreements. “Diplomatic recognition of the People’s Republic of China, the resolution of the Iran hostage crisis, and the conclusion of the Vietnam War were all conducted without congressional approval,” he noted.

The Republican-drafted letter was an aggressive attempt to make it more difficult for Obama and five world powers to strike an initial agreement by the end of March to limit Iran’s nuclear program, which Tehran insists is for peaceful purposes.

Republicans worry that Iran is not negotiating in good faith and that a deal would be insufficient and unenforceable, allowing Iran to eventually become a nuclear-armed state. They have made a series of proposals to undercut or block it — from requiring Senate say-so on any agreement to ordering new penalty sanctions against Iran to threats of stronger measures.

The Republicans’ move comes just days after Israeli Prime Minister Benjamin Netanyahu spoke to a joint meeting of Congress at Republican House Speaker John Boehner’s invitation. In his address, Netanyahu bluntly warned the United States that a deal would pave Iran’s path to a nuclear bomb.

“I think it’s somewhat ironic that some members of Congress want to make common cause with the hard-liners in Iran,” Obama said about conservative Iranians who also are leery of, or downright against, the negotiations. “It’s an unusual coalition.”

The letter, written by freshman Sen. Tom Cotton, was addressed to the “Leaders of the Islamic Republic of Iran” and presents itself as a constitutional primer to the government of an American adversary.

Explaining the difference between a Senate-ratified treaty and a mere agreement between Obama and Iran’s Ayatollah Ali Khamenei, the senators warned, “The next president could revoke such an executive agreement with the stroke of a pen, and future Congresses could modify the terms of the agreement at any time.”

Iranian Foreign Minister Mohammed Javad Zarif was quoted by the website of Iranian state TV on Tuesday as saying the letter’s warning that any nuclear deal could be scrapped once Obama leaves office suggests the United States is “not trustworthy.” He called the letter “unprecedented and undiplomatic.” Earlier, he had dismissed it as a “propaganda ploy.”

Not all Republican senators are united. One significant signature missing from Monday’s letter was Bob Corker of Tennessee. As chairman of the Senate Foreign Relations Committee, he said he wants to focus on a bipartisan effort that can generate a deal.

“We’ve trying to lead a solid bipartisan effort that tries to generate an outcome,” Corker said.

Negotiating alongside the U.S. are Britain, China, France, Germany and Russia. Nuclear negotiations resume next week in Switzerland.

Officials say the parties have been speaking about a multi-step agreement that would freeze Iran’s uranium enrichment program for at least a decade before gradually lifting restrictions. Sanctions relief would similarly be phased in.

The Obama administration believes it has authority to lift most trade, oil and financial sanctions that would be pertinent to the nuclear deal in exchange for an Iranian promise to limit its nuclear programs. For the rest, it needs Congress’ approval. And lawmakers could approve new Iran sanctions to complicate matters.

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Associated Press writers Alan Fram, Steve Peoples, Chuck Babington, Laurie Kellman and Jim Kuhnhenn in Washington and Cara Anna at the United Nations contributed to this report.