John Bolton, an Undiplomatic Voice for American Might

Critics, however, expressed concern that a bomb thrower in words could become a bomb thrower in deeds. “Bolton played a key role in politicizing the intel that misled us into the Iraq War,” Senator Edward J. Markey, Democrat of Massachusetts, wrote on Twitter. “We cannot let this extreme war hawk blunder us into another terrible conflict.”

Clifford Kupchan, the chairman of the Eurasia Group, a consultant firm, sent a note to clients warning that Mr. Bolton’s appointment “increases risk across the board,” adding that it “makes U.S. foreign policy ‘America First on Steroids.’”

At one point, Mr. Bolton toyed with running for president himself, only to back off. Instead, he created an organization to support like-minded candidates. His “super PAC” was one of the earliest customers of Cambridge Analytica, which has found itself confronting a deepening crisis after reports last weekend that the firm had harvested the data from more than 50 million Facebook profiles in its bid to develop techniques for predicting the behavior of individual American voters.

The firm was founded with a $15 million investment from Robert Mercer, the wealthy Republican donor who has backed both Mr. Bolton’s PAC and Mr. Trump. Cambridge’s so-called psychographic modeling techniques, which were built in part with the data harvested from Facebook, underpinned its work for the Trump campaign in 2016, setting off a furious debate about the merits of the firm’s methods. The same techniques were also the focus of its work for Mr. Bolton’s PAC.

Using the psychographic models, the company designed advertisements from candidates supported by Mr. Bolton’s PAC, including the 2014 campaign of Thom Tillis, the Republican senator from North Carolina. One advertisement, a video that was posted on YouTube, was aimed at fearful and neurotic voters — it emphasized security and the idea that Mr. Tillis could keep the United States safe.

Mr. Bolton also recorded a video used by a Russian gun rights group in 2013 to encourage Moscow to loosen gun laws, according to a report by NPR. The report said the video was part of an effort by Russian and American gun rights groups to collaborate in the years leading up to the 2016 election.

A native of Baltimore, Mr. Bolton, 69, received undergraduate and law degrees from Yale University. In between stints in private practice, he took a series of increasingly important jobs in government, starting at the United States Agency for International Development under President Ronald Reagan and later as an assistant attorney general.