Washington (CNN) President Barack Obama slammed Republican senators who penned a letter attempting to warn Iran that any pending nuclear agreement will face their scrutiny, claiming they were aligning themselves with Iranian "hard-liners."

"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."

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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.

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Sens. Jeff Flake, Lisa Murkowski, Lamar Alexander, Dan Coats, Thad Cochran, Susan Collins and Bob Corker didn't sign the letter. Murkowski and Coats are the only two up for re-election, and Coats is rumored to be considering retirement.

Collins said while she agrees any Iran agreement should come before the Senate, this letter won't likely sway negotiations.

"I don't think that he Ayatollah is going to be particularly convinced by a letter from members of the Senate even when signed by a number of my distinguished and high-ranking colleagues," she said on Monday.

Flake's spokeswoman Bronwyn Lance Chester said the senator agreed with the spirit of the letter, but abstained from signing it because he did not "believe the letter was necessary."

Corker, who's chairman of the Senate Foreign Relations Committee and has been pushing a bill to require congressional review of any deal, indicated that measure was his most pressing concern.

"As chairman of the Senate Foreign Relations Committee, Sen. Corker's focus is on getting a veto-proof majority to support his bipartisan bill for congressional review of any comprehensive nuclear agreement with Iran," an aide said.

Speaking on Fox and Friends on Monday morning, Cotton again insisted that congressional approval is necessary for any deal to last.

"Any deal that is not approved by the Congress won't be accepted by the Congress now or in the future," he said, adding that he hopes more lawmakers and presidential candidates sign on.

But he indicated that congressional approval might be tough to get, if the developing contours of the deal remain intact.

The Arkansas Republican said that the terms of the deal, including Iran's robust uranium enrichment capability and the possibility of a sunset in as little as ten years, make it "unacceptable, dangerous to the United States, and dangerous to the world."

Cotton appears to have slightly flubbed the wording on treaty ratification, however — according to a 2001 Congressional Research Service Report , "it is the President who negotiates and ultimately ratifies treaties for the United States."

The Constitution does stipulate that the Senate plays a role in that process, however, by taking up a "resolution of ratification" that must pass with a two-thirds majority.

Zarif also claimed the GOP senators were ignorant of their own Constitution.

"This proves that [the senators] are not only strangers to the norms of international rights and regulations, but they also are not familiar with the intricate details of their own Constitution regarding the authority of the President of the United States in executing foreign policy," he said.

Despite the imprecise wording, the letter is intended to pressure the Obama administration to give Congress final approval over the developing deal with Iran over its nuclear program. A bipartisan group of senators is currently working to usher a bill to do just that through the Senate, but Democrats have said they won't move forward with the measure until the first deadline for the talks to bear fruit, at the end of this month.

The warning could have the added effect of further complicating already delicate talks between the two nations aimed at reigning in Iran's nuclear program.

Democrats on Monday accused Republicans of attempting to do just that, with White House press secretary Josh Earnest calling the letter a "continuation of a partisan strategy to undermine the President's ability to conduct foreign policy."

"To essentially throw sand in the gears here is not helpful, and is not, frankly, the role our founding fathers envisioned for Congress to play when it comes to our foreign policy," he said.

Senate Minority Leader Harry Reid disparaged Cotton's letter on the Senate floor, saying it was aimed at "sabotaging" the Iran talks.

"This letter is a hard slap not only in the face the United States, but our allies," Reid said, as Cotton -- who happened to be presiding over the Senate -- looked on. "This is not a time to undermine the commander in chief purely out of spite."

Senate Minority Whip Dick Durbin agreed, calling it a "cynical effort by Republican senators to undermine sensitive international negotiations" in a statement.

"It weakens America's hand and highlights our political divisions to the rest of the world," he said.

"Understand that if these negotiations fail, a military response to Iran developing their nuclear capability becomes more likely," Durbin said. "These Republican senators should think twice about whether their political stunt is worth the threat of another war in the Middle East."