From the Middle of the Twentieth Century on Through the Obama

From the middle of the twentieth century on through the Obama administration, one issue that marked the Republican Party, and its "brand," was its championing of free trade.Democratic presidents of the same era also backed free-trade policies, but party members in Congress were more divided on the matter.
During the 2016 election, however, Donald Trump ran for the Republican nomination on a platform that included opposition to free-trade agreements, and support for import tariffs.About a year-and-a-half later, as president, he carried through on his talk, laying tariffs against Chinese exports, as well as those from western, democratic state allies.And polling showed that a majority of people identifying as Republican supported his tariffs, despite opposition to the tariffs among GOP congressional leaders.Meanwhile, polling showed that a majority of Democrats opposed the tariffs.
What does such a change in public opinion say about the nature of partisan identification and how parties evolve or change over time?