WASHINGTON -- The start of the NAFTA renegotiation process is working through a final hurdle in the U.S. Senate -- one imposed by a famous member of President Donald Trump's own party, in a symbolic example of the free-trade rift between the past and present of the GOP.

The party leadership began closure proceedings late Tuesday to force a vote on the confirmation of Trump's trade czar. That means a few more days of debate, followed by Robert Lighthizer's likely confirmation, which would let him start preparing for negotiations later this year with Canada and Mexico.

One reason it didn't happen sooner: unanimous consent was denied by John McCain.

The Republican icon and onetime presidential nominee dismissed suggestions he's been nursing a grudge. Lighthizer wrote a mildly critical newspaper column in 2008 that said McCain's support of free trade didn't prove he was conservative.

Rather, McCain said, it's a matter of principle. He said he has been asking questions, and awaiting answers from Lighthizer, about free trade, an issue central to the Republican party orthodoxy in the pre-Trump era.

"It has a lot to do with whether a trade representative is for free trade or not. Or whether they want to be further isolationists and destroy our economy," McCain said Tuesday on Capitol Hill.

"I'm not saying I'm opposing him. I want answers to the questions. That's what senators do. That's the right of a senator. It's normal. It's what we do every day."

The administration desperately hopes to get moving on NAFTA. Once Lighthizer is confirmed, it could file its 90-day notice before starting negotiations. Commerce Secretary Wilbur Ross says he hopes Lighthizer gets confirmed as early as this week.

Ross said there's mounting time pressure. With the Mexican election approaching and the U.S. midterm elections shortly thereafter, both the Mexican and American administrations have said they want a deal by early 2018.

Failure to seal an agreement by then could delay things to 2019.

"The Senate has been slow-walking the confirmation of Bob Lighthizer," Ross told a conference at the State Department. "We will seek a far more aggressive meeting schedule (on NAFTA this fall) than has been the norm thus far."

He repeated his frequently stated position that he's willing to negotiate a bilateral or trilateral deal, and isn't particularly wedded to either approach.

Lighthizer's confirmation should be a fait accompli, once it's up for a vote. Many Democrats sound ready to support him. The progressive, trade-skeptical Sherrod Brown of Ohio said he's spoken to Lighthizer four times since his confirmation hearing, really likes him, and can't understand what's taking so long.

"I think he's one of the best -- maybe the best -- single Trump nominee so far," Brown said. While Democrats had earlier held up the nomination over a procedural issue, Sen. Claire McCaskill said it's Republicans delaying now: "They're having trouble on their side of the aisle. It's not us."

This bizarre role-reversal reflects the fact that, in U.S. politics, attitudes to trade don't follow neat partisan lines.

And whatever lines existed are shifting in the Trump era, according to a new poll.

A survey conducted in the runup to the renegotiations of NAFTA finds majority support for the agreement in all three countries involved. The Pew Research survey finds that of those who took part in the poll, 76 per cent of Canadian respondents, 60 per cent of Mexicans and 51 per cent of Americans supported NAFTA.

But there's turbulence under the surface of U.S. public opinion.

The poll suggests a massive gap in partisan attitudes, with only 30 per cent of respondents who voted Republican supporting NAFTA compared with 68 per cent of Democrats. That's the opposite of the partisan breakdown of a few years ago -- and suggests a major Trump effect.

Republicans are suddenly the more trade-skeptical party, in keeping with Trump's attitude and shifting away from the views of Senate Republicans -- like McCain.

The veteran senator finished the day with a forceful defence of NAFTA.

In a panel discussion at the U.S. State Department, he shared memories of its 1992 signing, describing a moving speech by Brian Mulroney and a celebration with Mexico's president. He said it has benefited all countries, now supports 200,000 jobs in his home state of Arizona, and has given the U.S. a great gift: a more stable southern neighbour, with a growing middle class.

"I'm sorry I get so passionate about it," McCain said. "I happen to believe in free trade."