“I led a combat unit that killed a large number of people,” he boasted in a 2019 interview.

He has long advocated an imperialist foreign policy. In 2021, he was elected to Russia’s Parliament — a sign of the nationalists’ rising stature in President Vladimir V. Putin’s system — but gave up his seat. Russian media speculated at the time that Mr. Prilepin might have presidential ambitions and that he sought a role that was more high profile than as one of the hundreds of members of Russia’s rubber-stamp State Duma.

He found that role after Mr. Putin’s full-scale invasion of Ukraine in February 2022, when the aggressively imperialist views that had earlier put Mr. Prilepin on the edge of Russia’s political mainstream became the Kremlin’s guiding ideology. On Telegram, where Mr. Prilepin has more than 300,000 followers, he was a vocal cheerleader of what the Kremlin has termed the “special military operation.”

On Monday, he posted an archival photo of Soviet-era celebrations in Kyiv of May 1, the Communist Labor Day holiday, and wondered why people in the Ukrainian capital were no longer celebrating.

“Are they waiting?” he asked. “Are they dead? Did a demon inhabit them?”

After the car bombing in August that killed Ms. Dugina on her way home from a literary festival he hosted, Mr. Prilepin was quoted as saying that the West had “habituated” Ukraine to such actions.

U.S. intelligence agencies believe that parts of the Ukrainian government authorized the car bomb attack that killed Ms. Dugina. American officials have said they were not aware of the operation ahead of time and would have opposed the killing had they been consulted.

Cassandra Vinograd and Anastasia Kuznietsova contributed reporting.