"It merely clears a field for the locals…"
Unfortunately for the USA, the country has been endlessly using and abusing protectionism - either directly with tariffs (light trucks and agricultural products ) or indirectly with technical rules such as the USA(LHD) vs UN(LHD) carmaking rules over the last 70 years to insulate local producers from world realities that the internal market is completely and utterly warped in its views of everywhere else.
Carmakers are screaming that having to make UN(LHD) standard vehicles is a trade barrier erected against them. Light truck makers are screaming that noone wants to buy their overpriced, oversized, overpowered, unreliable and thirsty vehicles outside the USA (except as boutique purchases). Farmers don't really care because they have a captive market that for the most part doesn't notice that the steaks are grey, not red, or that food poisoning traceable to chicken slaughtering is 100 times the level in the EU. Telcos have somehow managed to obtain abusive monopolies without the pesky Universal Service committments that AT&T used to have as a result of the 1932 FTC Antitrust settlement.
The USA is large enough to be a self-contained market, but the barriers it's erected against others entering tend to grate when it's demanding access everywhere else and demanding "free trade or else!"
Tariffs haven't been clearing fields for locals in the USA for a very long time. They're been allowing the incumbents to maintain their cushy lifestyles by shutting out new entrants - and whilst these kinds of misuses are the very thing that GATT and WTO rules are intended to try and tear down, the USA has been erecting more and more such barriers of late.
One thing that Trump is right about - he really is "Making America Grate"