Any doubts that Donald J. Trump has had a huge (as he might say) influence on the Republican Party were dispelled this month when Senator Rob Portman came out against the Trans-Pacific Partnership.

There is no more respected member of what’s called the Republican establishment: Senator Portman, an Ohio lawmaker, is a confidant of the Bush family, a runner-up vice-presidential pick in 2012 and a former United States trade representative.

But Mr. Portman is up for re-election, and Mr. Trump has changed the dynamics of the trade debate. Mr. Portman voted last year to give President Obama fast-track negotiating authority on trade agreements, which was intended to pave the way for pushing the 12-nation Trans-Pacific Partnership through Congress. But this month he said he was opposed to the agreement.

Republicans used to be the protectionist party. They wrote the infamous Smoot-Hawley Tariff Act of 1930, which raised tariffs on imports. After World War II, when the United States emerged as a global superpower, both parties essentially embraced the nation’s role as a leader in the world’s economic recovery.