President Barack Obama has expressed his embarrassment for the Republican senators who tried to circumvent his authority in executing foreign policy this month by warning in an open letter to Iran's leaders that any nuclear deal their government signs with the president could be annulled after Obama leaves office.

The president's comments were made during an exclusive interview with VICE founder Shane Smith on Tuesday, in which Obama discussed a host of issues important to Americans, from Washington politics and foreign policy to college affordability and global warming. The full interview will be posted to VICE News on Monday, March 16.

On the topic of Iran, Obama said that the senators' intrusion into sensitive foreign diplomacy and their evident lack of respect for the role of the presidency is "not how America does business."

"I'm embarrassed for them," the president remarked of the letter's original 47 signatories, which included the Republican Senate leadership as well as potential 2016 presidential candidates Ted Cruz (Texas), Marco Rubio (Florida), and Rand Paul (Kentucky). "For them to address a letter to the ayatollah — the supreme leader of Iran, who they claim is our mortal enemy — and their basic argument to them is: don't deal with our president, because you can't trust him to follow through on an agreement... That's close to unprecedented."

On Tuesday, more Republicans came out in support of the letter, among them former Texas Governor Rick Perry, Pennsylvania Senator Rick Santorum, and Louisiana Governor Bobby Jindal, whose name has since been formally added to the list of backers.

Members of the GOP caucus have sought to undermine Obama's effort to secure a deal with Iran over the Islamic Republic's nuclear program. Earlier this month, Israeli Prime Minister Benjamin Netanyahu delivered a speech before Congress arguing against the deal being negotiated in Geneva. House Speaker John Boehner had invited him to deliver the remarks without first consulting the Obama administration.

The Republican lawmakers' open letter stressed that a nuclear agreement made without Congressional approval might not last.

"It has come to our attention while observing your nuclear negotiations with our government that you may not fully understand our constitutional system," the letter said. "Anything not approved by Congress is a mere executive agreement… 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."

Ayatollah Ali Khamenei, Iran's supreme leader, responded to the open letter in a statement posted on his website Thursday, saying that international foreign policy protocol prescribes that agreements are honored even when governments change guard. The subordination and "backstabbing" exhibited by the American lawmakers is an example of Washington's use of "tricks and deception," which "worries us," the leader said.

"US senators officially announce that when this government leaves, its commitments will become nullified," he said. "Isn't that the ultimate collapse of political ethics and the disintegration of the US system?"

Despite the senators' letter, and opposition from Iranian hardliners who oppose the deal, the leader has not backed away from negotiations.

Obama will continue to work toward securing a deal with Iran, in the face of what he described to VICE as the GOP's "slash-and-burn approach to politics."