From the day he rode down an escalator at Trump Tower to launch his campaign, Donald Trump’s candidacy was identified most often with one issue: fighting illegal immigration.

A deeper look into the results of the election that propelled him to the presidency, however, suggests the real fuel behind his victory may have come more from his stand against traditional free-trade agreements, and in his overall call for “change.”

The distinction is important, because it will help determine where Mr. Trump will have the broadest political support for policy change once he occupies the Oval Office he visited Thursday.

In any presidential transition, figuring out the difference between campaign rhetoric and governing reality is tricky. The distinction is driven both by what an incoming president really meant in campaign talk, but also by what kind of mandate can actually be read into the vote.

In this election, the decisive factor—the one that actually was the difference between a Trump victory and a narrow Hillary Clinton one—was the surprising outcome in the old industrial states of the upper Midwest: Pennsylvania, Ohio, Michigan and Wisconsin. Wisconsin hasn’t gone Republican since 1984; Pennsylvania and Michigan since 1988; Ohio since 2004.