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.”