WASHINGTON — Democrats in the White House and Congress accused 47 GOP senators of undermining President Barack Obama in international talks to curb Iran’s nuclear program, saying that trying to upend diplomatic negotiations was tantamount to rushing into war with Tehran.

“The decision to undercut our president and circumvent our constitutional system offends me as a matter of principle,” Vice President Joe Biden said in a statement.

In an open letter Monday to the leaders of Iran, Republican lawmakers warned that any nuclear deal they cut with Obama could expire the day he walks out of the Oval Office. The 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. Senate Majority Leader Mitch McConnell of Kentucky’s signature is on it, as are those of several prospective presidential candidates.

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.”

Cotton defended the letter in a series of television appearances Tuesday morning, denying emphatically that it undermines Obama’s negotiating position with Iran.

“No,” 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.”

Appearing on MSNBC, Cotton said Iran “has a very clear and simple path,” calling on Tehran to renounce nuclear weapons, disarm and allow unobstructed, unimpeded international inspections. The Arkansas Republican accused Iran of seeking “a nuclear umbrella so they can continue to export terrorism around the world.”

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.”