As President Donald J. Trump battled public outrage in the summer of 2019 over his effort to enlist Ukraine in digging up dirt on former Vice President Joseph R. Biden Jr., he summoned a small group of top advisers to the Oval Office. Among them was Charles Kupperman, the deputy national security adviser, who was surprised to see a relatively low-level staff member in one of the four chairs arrayed before the president’s desk: Kashyap “Kash” Patel.

Mr. Patel, a newcomer to the National Security Council staff from the House Intelligence Committee, had impressed the president as the primary author of the secret “Nunes memo,” a key element in the effort of House Republicans to undermine the Justice Department’s investigation of Russian meddling in the 2016 election. Now Mr. Trump was suggesting an additional role for Mr. Patel.

“He wanted to make Kash a political executioner, to root out and fire individuals on the White House staff who weren’t being as loyal as he thought they should be,” Mr. Kupperman said in a recent interview.

Alarmed, Mr. Kupperman pushed back, as did Pat Cipollone, the White House counsel, and John Eisenberg, the National Security Council’s legal counsel, who were both there that day. All three said that loyalty tests would create legal and morale problems.

Eventually Mr. Trump stood down. But Mr. Kupperman today sees the incident as a warning of Mr. Trump’s intention to stock a second administration with people like Mr. Patel: valued more for subservience than expertise, and eager to pursue a vengeful president’s whims.

“Trump’s people are concerned with having a very weak civil service who are just automatons, loyal to him,” he said, referring to Mr. Trump’s pledge to reclassify tens of thousands of executive branch employees in a way that would enable the president to fire them. “Kash would have done it in a heartbeat.”