WASHINGTON -- A number of pro-trade American lawmakers had lunch today with Donald Trump, ramping up the political campaign to prevent him from blowing up NAFTA.

Trump met at the White House with senior staff and a half-dozen Republican senators.

The lawmakers are urging him not to start cancelling NAFTA as Trump has threatened he might, as a bargaining tactic.

This comes after a meeting in which U.S. auto companies took their concerns to Vice-President Mike Pence, frustrated with the proposals the Trump team has put on the negotiating table.

Those proposals have been deemed non-starters by Mexico and Canada, with Ottawa's chief negotiator telling a parliamentary hearing this week that Canada wants to offer constructive counter-proposals but is wary of engaging on some terms presented by the U.S. side.

One of the senators present at today's meeting, Iowa's Joni Ernst, says she stressed to Trump's team the importance of maintaining NAFTA.