WASHINGTON — In Donald J. Trump’s private conversations and public commentary, one guiding principle shines through: The world is a zero-sum place, and nations, like real estate developers, are either on the winning side of a deal or the losing side.

Yet he also is the ultimate pragmatist, perfectly willing to dispense with seemingly core beliefs in return for negotiating advantage. That is why many of his closest supporters have long cautioned that the most headline-grabbing proposals of his run for the presidency should not be taken literally — they are guideposts, the supporters suggest, not plans. Even Mr. Trump once described his proposed ban on Muslim immigrants as a mere “suggestion.”

As he enters the Oval Office that Ronald Reagan — another populist pragmatist, but one who had served in public office before the White House — left nearly 28 years ago, the world is about to find out what Donald Trump really believes. Or at least what he is going to try to do, in partnership with Republicans who on Tuesday retained control of both houses of Congress.

It was in Mr. Reagan’s last months in office that Mr. Trump took out a full-page ad in several newspapers complaining that “for decades, Japan and other nations have been taking advantage of the United States.” Flirting with a presidential run himself — he was 41 — and seeking the publicity that would become addictive, he called for the United States to pull out of the Middle East, which he called “of only marginal significance to the United States for its oil supplies,” and asked, “Why are these nations not paying the United States for the human lives and billions of dollars we are losing protect their interests?”