No matter how difficult talks get, or how extreme the American proposals are, Canada has no plans to walk out during this round of NAFTA talks.

Mexico's president also indicated after his meeting with Prime Minister Justin Trudeau in Mexico City Thursday that his country will sit through this round of discussions, even though all sides expect tensions to rise.

American negotiators will present their most divisive NAFTA proposals this week, as representatives from the U.S., Canada and Mexico meet for the fourth round of talks, in Arlington, Va.

The U.S. will introduce its most protectionist demands yet, including changes to the rules around auto production to boost American content requirements, more access to the dairy market in Canada, and a sunset clause that would require all countries to recommit to NAFTA every five years.

These are proposals that Canada and Mexico have described as non-starters, and analysts had suggested they could trigger a theatrical walkout.

But a source close to the negotiating table downplayed that suggestion, saying negotiators are prepared to sit at the table through anything at this point.

Following his meeting with President Enrique Pena Nieto Thursday, Trudeau told reporters Canada remains committed to the talks.

"We will continue to take seriously the work we do and we will not be walking away from the table based on proposals put forward," Trudeau said. "We will discuss those proposals, we will counter those proposals, and we will take seriously these negotiations."

U.S. negotiating in earnest

The source also dismissed concern that the U.S. is not committed to seriously renegotiating NAFTA, saying the time and resources put into the process by the Americans have been significant.

The mood around the negotiating table has not been positive, according to stakeholders and sources.

Jerry Dias, the head of Unifor, Canada's largest private sector union, described the negotiations as being "in the toilet."

But CBC News has also learned that the prime minister got some reassurance from influential lawmakers during his two-day visit to Washington.

Members of the powerful congressional ways and means committee welcomed Justin Trudeau's pro-NAFTA message.

The source said support for the trade deal was overwhelming, and that both Democrats and Republicans in the committee were in favour of modernizing the agreement.

This is significant for Trudeau, since the committee has the ability to quash an attempt by President Donald Trump to kill NAFTA, if he makes good on his threats.

The source said Trudeau appreciated the support.