But then on Friday night, the Canadians, who had previously showered reporters with briefings, updates, notes of encouragement and even Popsicles, went silent. United States officials, who are notoriously less loquacious, also went totally dark.

Rather than negotiate on President Trump’s turf, Chrystia Freeland, Canada’s foreign minister (and, full disclosure, my former boss at Financial Times), scrapped her speech at the United Nations and hunkered down in Ottawa to huddle with Prime Minister Justin Trudeau. Hordes of Canadian reporters who had been camped out in sweltering Washington all summer were instead bemoaning the chilly autumn weather outside of Mr. Trudeau’s office.

By Sunday afternoon it was clear that things were getting serious.

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Congressional aides — the sieve of Washington — were getting briefed by the staff of Robert E. Lighthizer, Mr. Trump’s top trade adviser, and I started getting calls from lobbyists with fresh details.

In the early evening, a Mexican official sent me a text saying copies of the agreement would be delivered to Mexico’s Senate within hours. A colleague who covers the White House alerted me that Jared Kushner, a White House adviser who was on the United States negotiating team, skipped a Sukkot dinner to work on the deal’s finishing touches.