The sudden firing on Tuesday of Secretary of State Rex W. Tillerson, the former Exxon Mobil chief executive who never managed to capture the role of chief diplomat, makes room for a true believer in President Trump’s “America First” views and a bitter critic of the Iran nuclear deal — but also a deep skeptic about whether negotiations will persuade North Korea to abandon its nuclear arsenal.

Mr. Tillerson’s anticipated replacement, Mike Pompeo, the C.I.A. director, was among the harshest critics of the 2015 nuclear agreement that world powers brokered with Iran. If confirmed, Mr. Pompeo will take over the State Department just as the president is weighing whether to ditch the deal altogether — even if it outrages European allies.

The move would also put Mr. Pompeo, who has been immersed in the details of Pyongyang’s nuclear program, in a central role in running the negotiations with Kim Jong-un, the North Korean dictator whom Mr. Trump has said he will meet by May.

For all the criticisms of Mr. Tillerson — and there were many, particularly in the State Department as he moved to slash its size — he was considered a restraining influence on Mr. Trump. Mr. Pompeo, in contrast, has been an enthusiastic defender of the president’s policies, to the point that many senior current and former C.I.A. officials worried that he was far too political for the job.