In 2016, running for president and pressed for details on how he would handle some of the world’s knottiest security issues, then-candidate Donald J. Trump had a simple formula for defanging the Iranian nuclear program.

Barack Obama’s negotiating team, he said, should have just gotten up from the table and stormed out. The Iranians would have come begging. “It’s a deal that could’ve been so much better just if they’d walked a couple of times,” Mr. Trump told two reporters from The New York Times. “They negotiated so badly.”

Now, at a moment the Iranians are far closer to being able to produce a weapon than they were when the last accord was negotiated — in part because Mr. Trump himself upended the deal in 2018 — the president has his chance to show how it should have been done.

So far, the gap between the two sides appears huge. The Iranians sound like they are looking for an updated version of the Obama-era agreement, which limited Iran’s stockpiles of nuclear material. The Americans want to dismantle a vast nuclear-fuel enrichment infrastructure, the country’s missile program and Tehran’s longtime support for Hamas, Hezbollah and other proxy forces.

What is missing is time.

“It is essential that we reach an agreement quickly,” said Senator Jeanne Shaheen of New Hampshire, the top Democrat on the Foreign Relations Committee, who called Mr. Trump’s decision to pull out of the Iran nuclear deal a “serious mistake.” “Iran’s nuclear program is advancing every day, and with snapback sanctions set to expire soon, we are at risk of losing one of our most critical points of leverage.”

Snapback sanctions allow for the quick reimposition of United Nations sanctions against Iran. They are set to expire Oct. 18.