WASHINGTON — President Trump has spent months pummeling the North American Free Trade Agreement, calling the trade pact a “disaster” and insisting that it cost American jobs. Now, United States trade negotiators will have to decide how tough to get on their Nafta counterparts.

As Canada, Mexico and the United States reconvene in Ottawa this weekend for a third round of trade talks, they are plunging into some of Nafta’s thorniest issues.

It is not clear that the trading relationship will emerge intact.

The talks, which have centered on logistics and topics of agreement, are now heading into the gritty details of proposals that could ultimately upend the pact and force any of the trading partners to walk away from the deal.

The American government is prepared to come out swinging. Wilbur Ross, the secretary of commerce, wrote in an op-ed in The Washington Post on Thursday that Nafta has provided too many advantages to workers and countries outside the North American pact and put American jobs at risk.