President Trump waves after speaking to world leaders at the U.N. General Assembly in New York on Tuesday. (Spencer Platt/Getty Images)

The Trump administration is unlikely to brief senators about the Iran nuclear deal until the president has decided whether Tehran is complying with its terms, according to correspondence cited by a senior congressional aide.

Senior Senate Democrats have been urging the administration to send the secretary of state, the director of national intelligence, the defense secretary and the chairman of the Joint Chiefs of Staff, and potentially the energy and treasury secretaries, to Capitol Hill sometime before Oct. 6 to brief senators about Iran's behavior under the nuclear deal. That is the last day senators will be in Washington before President Trump's Oct. 15 deadline to certify whether Tehran is compliant.

But in correspondence with congressional leaders, senior National Security Council officials said it would be "highly unlikely" that they could orchestrate such a briefing before late October, or even early November.

That is potentially problematic. If Trump determines that Iran is not compliant with the terms of the deal, it kicks off a 60-day window in which Congress must decide whether to reimpose the sanctions that were waived as part of the agreement. The move would effectively kill the deal and could put Tehran on a path toward obtaining a nuclear weapon.

The prospect of Iran resuming its nuclear work might seem a compelling reason for lawmakers to refuse to reimpose sanctions. But the Iran deal has never been popular in Congress, where more than half of the members opposed it coming into force in early 2016 and lawmakers continue to be angered by Iran's recent ballistic-missile tests.

Many Republican politicians campaigned against the Iran deal during the last election cycle — including Trump, who promised to rip it up on Day One. But he has not fulfilled that promise, opting instead to periodically extend sanctions relief to Iran.

Earlier this month, Trump decided again to waive nuclear sanctions against Iran. But during his speech to the U.N. General Assembly this week, he was dismissive of the nuclear agreement, calling it "an embarrassment" and "one of the worst and most one-sided transactions the United States has ever entered into."

"I don't think you've heard the last of it," he added. "Believe me."

Trump announced this week that he has made a decision about whether to certify Iran's compliance with the deal. But he would not say what that decision is, promising reporters only that he would let them know.

Senate Foreign Relations Committee Chairman Bob Corker (R-Tenn.) said last week that he was aware of the direction Trump was leaning, but "that could change," he said He also would not specify what he knew of Trump's thinking but added that "we are definitely preparing" in Congress for the aftermath of that decision.

"We're working hand in hand with the White House and with the State Department to be prepared, to be prepared to deal with this issue depending on the outcome," Corker said.

But most members of Congress are operating blindly.

The Trump administration has never briefed Congress behind closed doors about Iran's compliance with the deal, a point that ranking Democrats on several Senate committees made in a Wednesday letter to administration officials. They noted that the only administration official to have testified to a congressional panel about the Iran deal was Air Force Gen. Paul Selva and that "he noted that U.S. intelligence demonstrates that Iran is abiding by its commitments under the JCPOA," an acronym referring to the nuclear deal.

That could complicate lawmakers' decision-making should they be left to determine whether to reimpose sanctions on Iran.

The administration cited scheduling conflicts for the Cabinet-level officials whom lawmakers want to hear from, as well as the frequency of other briefings that administration officials are holding on the Hill, as reasons for the difficulty in scheduling an Iran briefing earlier.

Earlier this month, Secretary of State Rex Tillerson, Defense Secretary Jim Mattis, Joint Chiefs of Staff Chairman Joseph F. Dunford Jr. and Director of National Intelligence Daniel Coats came to Capitol Hill to brief senators and House members about Afghanistan and North Korea. The administration is also attempting to schedule an Oct. 4 briefing for members about the Foreign Intelligence Surveillance Act, according to correspondence cited by the senior congressional aide, who was not authorized to speak about the Iran issue on the record. The law that authorizes the federal government to conduct wiretaps on foreign agents located overseas is expiring at the end of the year, absent an extension by Congress.

Should Congress have to make a decision on Iran, it will be just the latest controversial policy fight lawmakers will be forced to engage in before a December deadline. In addition to the FISA Amendments Act extension, Congress must come up with a budget to keep the government funded and possibly strike an agreement to raise the debt ceiling.