"I think it's somewhat ironic to see some members for Congress wanting to make common cause with the hard-liners in Iran. It's an unusual coalition," Obama said Monday ahead of a meeting with European Council President Donald Tusk.

"I think what we're going to focus on right now is actually seeing whether we're going to get a deal or not. Once we do, if we do, we'll be able to make the case to the American people, and I'm confident we'll be able to implement it," Obama said.

Nearly every Senate Republican has signed on to an open letter to Iran's leaders warning that without their approval, any Iran nuclear deal signed by Obama will be null and void after he leaves office.

But a top Iranian negotiator and Democrats slammed the letter, calling it a purposeful attempt to undermine the delicate negotiations as they reach a pivotal deadline later this month.

"We believe this letter has no legal value and is indeed just a propaganda ploy," said Iranian Foreign Minister Javad Zarif, in a statement provided to and translated by CNN. "Whats more, while the negotiations have not yet borne fruit and there no agreement yet, pressure groups in the U.S. are so worried that they are using extraordinary measures to prove that they, just like Netanyahu oppose any kind of agreement."

Vice President Joe Biden joined in the chorus of voices speaking out against the letter, which he decried as "expressly designed to undercut a sitting President in the midst of sensitive international negotiations" and "beneath the dignity of [the Senate,] an institution I revere" in a statement.

"This letter sends a highly misleading signal to friend and foe alike that that our Commander-in-Chief cannot deliver on America's commitments—a message that is as false as it is dangerous," Biden said.

"Honorable people can disagree over policy. But this is no way to make America safer or stronger."

The letter, authored by Arkansas Sen. Tom Cotton, states that the Constitution requires any international treaty to be ratified by a two-thirds vote in the Senate, and "anything not approved by Congress is a mere executive agreement." It also notes that presidents are barred from serving more than two terms in office and that the Obama administration ends in 2017.

"What these two constitutional provisions mean is that we will consider any agreement regarding your nuclear-weapons program that is not approved by the Congress as nothing more than an executive agreement between President Obama and Ayatollah Khamenei," the senators write. "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."

The letter is signed by 47 Republican senators, including every member of GOP leadership and all four the the chamber's potential presidential contenders: Sens. Rand Paul, Ted Cruz, Marco Rubio and Lindsey Graham.

Cotton defended the letter during an appearance on CNN's "The Lead" on Monday.

"I support a good deal that stops Iran from getting a nuclear weapon today, tomorrow, 10 years from now, and forever," he told host Jake Tapper.

(CNN)