As the administration begins to resemble a game of reverse musical chairs — too many open slots without enough loyalists to fill them — a short list of plausible replacements emerged.

The acting Adviser

Charles M. Kupperman

Mr. Kupperman, a former Reagan administration official and defense contracting executive, is a longtime Bolton associate. Known by many national security officials by his nickname, “Kupperware,” for his blandness, Mr. Kupperman, 68, was appointed in January as deputy national security adviser under Mr. Bolton.

Shortly after Mr. Bolton left the White House on Tuesday, Hogan Gidley, a deputy White House spokesman, told reporters that Mr. Kupperman would serve as Mr. Bolton’s acting successor. Acting officials have a way of sticking around in this administration for indefinite lengths of time, but Mr. Kupperman’s track record as someone ensconced in Mr. Bolton’s inner circle could shorten his tenure.

Still, the president appreciated Mr. Kupperman’s just-the-facts style compared with Mr. Bolton’s often ideologically charged delivery: If Mr. Trump had to have a national security brief concerning long-term planning, he preferred it from Mr. Kupperman as opposed to Mr. Bolton, according to a person with knowledge of that process.

The representative to North Korea

Stephen E. Biegun

Mr. Biegun, the United States’ special representative for North Korea, had a firsthand window into the clashes between Mr. Bolton, who never wavered from a hawkish, hard-line stance on North Korea, and the president, who has tried to use a charm offensive to persuade Kim Jong-un, the North Korean leader, down a path to denuclearization.