WASHINGTON — President Trump said Monday that he planned to reveal his final decision on whether the U.S. should remain in the Iranian nuclear agreement on Tuesday, just days ahead of the May 12 deadline.

"I will be announcing my decision on the Iran Deal tomorrow from the White House at 2:00 pm," Trump said in a tweet.

Trump has until Saturday to decide whether to renew a 120-day waiver of sanctions that Congress previously imposed on Iran. If he does not renew the waiver, it would signal to U.S. allies and Iran that the U.S. wants to pull out of the nuclear deal negotiated under President Barack Obama. The agreement, to which the U.S., U.K., Russia, France, China and Germany are parties, lifted sanctions on Iran in return for limitations on that country's nuclear program.

Before Trump's tweet about his upcoming announcement, some Republican lawmakers — including longtime critics of the deal — had been urging him to stick with the agreement and try to change it, not withdraw completely.

Sen. Lamar Alexander, R-Tenn., who previously opposed the nuclear deal, said in an interview Monday on Hugh Hewitt’s radio show that he hopes that Trump can get a "better deal" and advised against pulling out of the agreement.

"When our country, even if it's just the president, makes an agreement with our allies with another country, we ought to be very careful about changing that," said Alexander, who added that while the deal had "a lot of flaws," it does provide "a window into what Iran is doing and limits their development of nuclear weapons."

"I would counsel against" pulling out, Rep. Mac Thornberry, R-Texas, chairman of the House Armed Services Committee, said in an interview on "Fox News Sunday."

Thornberry, who initially opposed the agreement, now says he wonders what would happen if the U.S. abandons it.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

"Secretary (of Defense James) Mattis talked about the inspectors that are in there. Does Iran kick those inspectors out so that we lose what visibility we have there?" he asked. "You need to have a clearer idea about next steps if we are going to pull out, and especially given the larger context of Iran's aggressive activities in the Middle East."

Last week, Sen. Bob Corker, R-Tenn., chairman of the Senate Foreign Relations Committee, said he had spoken with Trump and that the president had indicated that if nothing changes, he's "definitely" leaving the agreement.

French President Emmanuel Macron told reporters late last month, after meeting with Trump, that he believes the president "will get rid of this deal on his own, for domestic reasons."

Other Republicans say they are hoping that Trump modifies the agreement so that it addresses certain holes, such as Iran's ballistic missile program.

Sen. Mike Lee, R-Utah, who railed against the agreement in 2015, now supports improving the deal and having it sent to the Senate for ratification as a treaty, his spokesman Conn Carroll, told NBC News on Monday. Obama chose not to allow the Senate to vote on the deal.

Sen. Roy Blunt, R-Mo., who blasted the deal, said in a recent interview with NBC's "Meet the Press" that he now agrees with Macron on the best way to approach the deal: Alter it, don't ditch it.

Meanwhile, other Hill Republicans continue to back a U.S. withdrawal.

"I've said it from the beginning — Obama's Iran deal is a flawed agreement and I had the opportunity to discuss its flaws in depth with Prime Minster Netanyahu last October," said Sen. James Inhofe, R-Okla., in a statement provided to NBC News on Monday, referring to Benjamin Netanyahu, the Israeli prime minister.

"We need concrete changes that would include anytime, anywhere inspections of non-declared sites, dismantling the development of advanced centrifuges, remove all sunset provisions, address Iran's ballistic missile program and halt actions that create regional instability," Inhofe said. "Without these meaningful, necessary changes, I fully support the president withdrawing from the Iran deal."

Lawmakers aren't alone in pressing the president as the deadline ticks closer: Israel has long pushed the Trump administration to kill the deal, while key Obama administration officials, including former Secretary of State John Kerry, and European leaders are still making moves in an attempt to salvage it.

To try and protect the deal, Kerry met recently with Iranian Foreign Minister Javad Zarif and German President Frank-Walter Steinmeier, according to a report in The Boston Globe.

NBC’s Andrea Mitchell reports that British Foreign Secretary Boris Johnson, who met with Secretary of State Mike Pompeo on Monday, was also planning to sit down with Vice President Mike Pence and White House national security adviser John Bolton, who has criticized the agreement.

Johnson, who wrote an op-ed in The New York Times on Monday titled "Don't Scuttle the Iran Nuclear Deal," said on MSNBC's "Morning Joe" on Monday that there are ways to improve the deal "without throwing away the heart" of the agreement.

"It is the best thing that we have at the moment," Johnson said. "It has stopped the Iranians getting a nuclear weapon, and we've got to ask ourselves: How else would we do that?"