I have seen several articles saying that Donald Trump is "to the left" of Hillary Clinton on trade (e. g., this one by Fareed Zakaria). The idea is that support for free trade is the conservative position, and that protectionism is the liberal (or leftist) position. This idea seems hard to square with the history of trade agreements. For example, NAFTA was negotiated mainly by the George H W Bush administration, then passed with strong support from the Clinton administration. It was not a party-line vote: in the Senate, Republicans voted for it by 34-10, Democrats against by 27-28. Supporters included Ted Kennedy and Phil Gramm; opponents included Paul Wellstone and Strom Thurmond.
But that's about political elites--what about the general public? In 2009, the Pew Research Center asked "In general, do you think that free trade agreements like NAFTA, and the policies of the World Trade Organization, have been a good thing or a bad thing for the United States?" Among people who said they were conservative or very conservative, 38% said "good thing" and 43% said "bad thing"; among those who said they were liberal or very liberal, 49% said good and 31% said bad (the rest weren't sure). Democrats were more favorable (44% good, 33% bad) than Republicans (39% good, 40% bad).
Does the conventional wisdom have it backwards? Pew also asked the question in 2008. At that time, Republicans and conservatives were more favorable. For example, 41% of Republicans said they were a good thing (44% bad thing), compared to only 30% of Democrats (55% bad thing). (Overall support for "good thing" was considerably lower in 2008 than in 2009--I'm not sure why).
There were some consistent patterns--people with more education were somewhat more favorable at both times, as were people in the highest income category (over $150,000). But whether support for free trade was a "liberal" or "conservative" position seems to depend on the party of the president. My interpretation is that most ordinary people, even well-informed people, don't have a very definite position--almost everyone is in favor of "free trade" as long as it's "fair trade," and almost no one knows enough about the details of particular agreements to be able to say if they are "fair trade." In recent history, presidents have consistently been in favor of free trade agreements, so the position gets associated with its most visible supporter--the incumbent president.
[Data from the Roper Center for Public Opinion Research]