Ivanka and Jared left the White House behind for a life in Miami. Eric is running the family business. Tiffany got married. Barron is a budding strategist at NYU.

Of all the Trump children, no one has stuck closer to their father’s side than Donald Jr., the president-elect’s eldest son.

The 46-year-old Mr. Trump has found political power and personal fortune in stoking the flame of the Make America Great Again movement his father started. He has an array of conservative-focused businesses, from a publishing company to a seven-figure annual podcasting deal. Forbes recently estimated Mr. Trump’s worth — largely built in the wake of his father’s political career after Jan. 6, 2021 — to be around $50 million.

For that reason, his associates say, he has no plans to join the administration. But he also understands what his siblings and several first-term administration officials learned the hard way: Trying to serve as a gatekeeper for his father is a politically perilous exercise.

In recent weeks, as the president-elect builds out his administration, his son has served as something of a loyalty scanner. As they review candidates at Mar-a-Lago in Florida, the president-elect is concerned with who looks good and who can deliver a message, people around them say. His son is focused on whether they mean what they are saying, and if they present a threat to the MAGA order.