CHICAGO (CBS/AP) — The White House and top Democrats in Congress have accused a coalition of 47 Republican senators of attempting to undermine President Barack Obama’s efforts to negotiate an end to Iran’s nuclear program.

U.S. Sen. Dick Durbin said the 47 Republican senators sunk to “a new low” in sending an open letter to the leaders of Iran, warning any deal they make with Obama to halt development of a nuclear weapon might be revoked by the next president, or altered at any time by Congress.

“It’s virtually unprecedented,” Durbin said. “Here we are in the middle of delicate negotiations. We’re down to the last few days. We’re trying to reach a point with our allies where Iran is going to stop any development of a nuclear weapon, to put inspectors on the ground, and 47 Republican senators elbowed their way into the room and say ‘Don’t waste your time. We get the last word on this, and we’re not going for it.’ It really is irresponsible, as far as I’m concerned.”

Durbin said he would not go so far as to call the letter an act of treason, as some critics have said.

“I can just tell you that the seven Republican senators who refused to sign it showed good judgment. They understand that we only have one president, one commander-in-chief, and we are in negotiations, sitting at the same side of the table with our allies, who’ve imposed sanctions on Iran,” Durbin said. “We want to show good faith, and find a way to avoid military confrontation in the Middle East. We want Iran to stop any development of a nuclear weapon, and this letter doesn’t help.”

Illinois Republican U.S. Sen. Mark Kirk was among the 47 GOP senators who signed the letter.

Durbin called the letter “a new low” in the ongoing division between Republicans and Democrats in Congress.

“We’ve had our battles over domestic issues. In fact, it took six months for the Republican majority to agree to even fund the Department of Homeland Security last week. But now they’ve decided to plunge into foreign policy, and to say to our allies around the world, ‘Don’t sit down at the negotiating table with the United States, because ultimately the majority in Congress – the Republican majority – has the last word,’” Durbin said.

The White House has accused Senate Republicans of attempting to undermine the president’s authority to conduct foreign policy. White House spokesman Josh Earnest has said any final deal reached with Iran would not be subject to congressional approval.

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

Officials have said 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 has said 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.

Senate Minority Leader Harry Reid said Republicans were driven by animosity toward Obama and unwilling to recognize that American voters had twice elected him president. Reid said that even at the height of Democrats’ disagreement about the war in Iraq with former President George W. Bush, they would not have sent a letter to former Iraqi leader Saddam Hussein.

“Republicans don’t know how to do anything other than juvenile political attacks against the president,” Reid said.

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.”

Senator Mark Kirk released a statement on the letter saying, “In exchange for allowing Iran to maintain the capabilities to build nuclear weapons, the Administration promises to ‘comprehensively lift’ as many as 14 Iran sanctions laws, many of which passed with bipartisan veto-proof majorities. There is no Constitutional authority granting a president unilateral power to repeal American law. Sanctions should not be weakened until Iran stops its nuclear weapons program, stops supporting and exporting terrorism, stops aggression against its neighbors, stops egregious human rights abuses, and stops threatening to annihilate Israel. A better deal, with bipartisan congressional backing, is the best insurance policy against a nuclear Iran, a destructive arms race, and war in the Middle East.”

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