There is certainly a path to an agreement. Trudeau would have to give ground on dairy, and Trump would have to agree to keep a binding arbitration system for disputes that the United States has long detested.

“The elements of a deal are on the table, or close to the table,” said Maryscott Greenwood, CEO of the Canadian American Business Council and a former U.S. diplomat.

The sense of urgency is spurred by elections in three countries. The November U.S. midterms could change the NAFTA dynamic, and Mexico is in a hurry to sign a NAFTA deal before its president leaves office Nov. 30. On top of that, an Oct. 1 vote in Quebec — Canada’s largest dairy-producing region — is “complicating” the negotiations, according to a top U.S. official. The official blamed “political posturing” by Canadian officials who do not want to be seen to be “giving away the store” ahead of the election.

The key players returned to the negotiating table on Tuesday, but there were no major breakthroughs.

Besides the substantive issues of dairy and dispute resolution, also hanging over the negotiations are Trump’s consistent threats to just leave Canada out and do a deal only with Mexico.

The president’s staunchest backers credit him with recognizing that he has more power in these negotiations, and being willing to use it.

“[Trump] understands that a $22 trillion economy versus a $1.5 trillion economy gives him all of the leverage,” said Rep. Kevin Cramer of North Dakota, Republican co-chair of the House Northern Border Caucus. “Not most of it, but all of it.”

Some U.S. lawmakers, including Republicans and Democrats, have insisted that ditching Canada in favor of a bilateral deal would violate U.S. fast-track law, and Trump has retorted by threatening to blow up the deal entirely.

If they can stick to the negotiating table and avoid rash moves to pull out, there are concessions that could lead to a deal. White House economic adviser Larry Kudlow literally spelled it out, when describing the remaining sticking point in one weekend interview: “M-I-L-K.”

At issue are a pair of changes to dairy policy: liberalizing an undetermined percentage of Canada’s mostly closed market, and rescinding a rule that has helped Canadian cheese-making proteins undercut U.S. competitors.

“I don’t think Canada will surrender its entire supply management [dairy] system,” Greenwood said. “But could Canada increase U.S. quota access to its market over time? ... Yes, I think that’s a possibility.”

Dispute settlement is the other big obstacle to a U.S.-Canada breakthrough. This measure, known as Chapter 19, is the mechanism that handles disagreements over punitive duties slapped on Canadian lumber, airplanes and newsprint. This cuts to the heart of the reason Canada sought a free-trade deal with former President Ronald Reagan in the first place, in the hope of escaping the persistent threat of punitive duties in the 1980s.

It was the hardest-fought issue of those original negotiations, and nearly blew up the talks several times, until an agreement on the final night of bargaining.

Trudeau argued a few days ago that Canada needs this protection in Chapter 19, now, more than ever, to fend off a Trumpian surge of protective duties: “We know we have a president who doesn’t always follow the rules as they’re laid out.”

Some of Trudeau’s critics say Chapter 19 is the wrong hill upon which to sacrifice Canada-U.S. free trade.

They question the constitutionality and effectiveness of the international panels, whose rulings the U.S. has occasionally ignored. And they argue that the U.S. domestic system, including the International Trade Commission, a quasi-judicial federal agency, has matured since the 1980s and can be trusted as a neutral arbiter of disputes: to wit, the duties on Canadian newsprint and Bombardier planes were swiftly, and effectively, squashed by ITC decisions earlier this year.

One thing Canada has going for it is an ongoing reservoir of goodwill.

Lawmakers from the northern and southern borders, left and right, business and labor groups — all have warned against freezing out Canada, leaving a dearth of support for a U.S.-Mexico-only deal.

According to a new POLITICO/Morning Consult Poll, a majority of Americans, 52 percent, favor keeping Canada in the pact, compared with 24 percent who favor excluding Canada.

A final challenge to a deal involves personalities of the negotiators Trump and Trudeau have assigned to the task.

The public dust-up between Trump and Trudeau at the G-7 is merely the tip of the frosty iceberg. There have also been some angry exchanges at the negotiating table between the countries’ NAFTA leads.

U.S. Trade Representative Robert Lighthizer — a lifelong trade wonk and economic nationalist — has, on more than one occasion, fumed at his Canadian counterpart over leaks. The Canadians have protested that some of the leaks he complained about, in fact, originated in the U.S.

Lighthizer’s Canadian counterpart, Chrystia Freeland, is a former journalist, writer and globe-trotting polyglot who has taken to casting aspersions in public speeches against the Trump worldview.

She even handed Lighthizer a book that appeared to suggest this worldview basically caused World War I.

One U.S. source who has been involved in the NAFTA negotiations said Freeland has caused friction with her U.S. counterparts.

But the source called Lighthizer a professional, and said the negotiating parties are managing to negotiate professionally.

Ben White, Anna Palmer, Doug Palmer and Luiza Ch. Savage contributed to this report.