Forty-seven Republican senators warned Iran's leaders on Monday that any nuclear deal needs congressional approval in order to last beyond President Obama's term, in a stark letter aimed at re-asserting lawmakers' role as talks near a key deadline.

In an open letter to Iranian leaders, Sen. Tom Cotton, R-Ark., and 46 other Republicans said they wanted to educate Iran about the U.S. Constitution. Namely, they pointed out that without congressional approval on a deal, all Tehran would be left with is a "mere executive agreement" between President Obama and Ayatollah Ali Khamenei.

"The next president could revoke such an executive agreement with the stroke of a pen," they wrote, "and future Congresses could modify the terms of the agreement at any time."

They added: "We hope this letter enriches your knowledge of our constitutional system and promotes mutual understanding and clarity as nuclear negotiations progress."

Though the letter is addressed to leaders in Tehran, it seems as much aimed at delivering a message to Obama.

Republicans and some Democrats want Congress to vote on any agreement, and are pushing a bill that would give Congress a say despite resistance from the White House.

Senate Majority Leader Mitch McConnell, R-Ky., though, has agreed to ease off a short-lived effort to fast-track that legislation, amid some Democratic concerns.

Notably, the Republican co-sponsor of that bill, Sen. Bob Corker, R-Tenn., did not sign the most recent open letter to Iran's leaders. No Democrats signed the letter, either.

White House spokesman Josh Earnest said Monday that a congressional vote is not what Obama envisions.

"The fact is the president does not envision substantial sanctions relief for Iran right at the negotiating table," Earnest said during the daily White House briefing.

The nuclear pact negotiators are working on does not require congressional approval because it is not a treaty, which would require a two-thirds majority Senate vote to be ratified. However, as the 47 Republicans noted in their letter, approval from a congressional majority would give the deal the force of a "congressional-executive agreement."

The U.S. and other nations are seeking a pact that would let Western powers verify that Iran will not obtain a nuclear weapon.

The next negotiations are scheduled for March 15, and wide gaps remain between the two sides. The deadline for an outline of a U.S.-Iranian deal is at the end of March.

Iran has said its nuclear program is peaceful and is aimed at producing energy.

There was no immediate Iranian government reaction to the letter or any discussion of it in Iranian media.

Cotton is a freshman senator who serves on the Senate's Armed Service and Intelligence committees.

The Associated Press contributed to this report.