WASHINGTON — For a diplomat, Richard Grenell has a remarkable record of being undiplomatic.

Facing rejection for a job a few years back at a global bank, he responded with a series of cutting email denunciations. Unhappy working as a foreign policy spokesman for Mitt Romney’s presidential campaign in 2012, he exited in a wave of recriminations.

His confrontational style touches such raw nerves that one top Obama administration official, Susan E. Rice, recently called him “one of the most nasty, dishonest people I’ve ever encountered,” and some German politicians demanded that he be recalled from his post as United States ambassador to Berlin. His career was built on media relations, but his Twitter attacks on journalists have led some to mute him or skip his briefings.

Bombastic perseverance and ostentatious attention-seeking may be problematic traits for some employers, but they have helped fuel Mr. Grenell’s rapid rise under President Trump.

In 2017, the president nominated him to be ambassador to Germany, a perch that Mr. Grenell has used to spread his brand of combative conservatism and emerge as an outspoken Trump loyalist. Last month, the president made Mr. Grenell the most powerful spymaster on earth, naming him the acting director of national intelligence for the United States — the least-experienced, most overtly political person ever to occupy the post, which oversees the Central Intelligence Agency, the National Security Agency and 15 other departments.