That leaves Mr. Borodai as a central figure in Ukraine’s immediate future. He may seem to have come out of nowhere, but in Russia he is a known quantity. He comes from a group of ultranationalists who were part of the far-right Zavtra newspaper in the 1990s. Their Pan-Slavic ideas, aiming for the unity of Slavic peoples, were considered marginal at the time. But they have now moved into the mainstream, helping formulate the worldview of today’s Kremlin, said Oleg Kashin, a Russian investigative journalist who has written extensively about Mr. Borodai.

“He’s the Karl Rove of Russian imperialism,” said Irena Chalupa, a fellow at the Atlantic Council.

When Mr. Borodai talks, people here listen. Surrounded by armed guards with scowling faces, Mr. Borodai stood with a microphone at the center of a large crowd that had gathered last weekend outside the compound of a local oligarch. They wanted to break in and declare it national property.

“I know many of you want a tour,” he said smiling, as the crowd cheered. “I respect that desire. But right now a tour is not possible.”

In an interview, Mr. Borodai said that he and Mr. Strelkov, the Russian rebel commander in Slovyansk, had both gone to Transnistria, a breakaway area in Moldova, to defend the rights of Russians in the 1990s. He named the cities in Russia that volunteers have come from, including Novosibirsk, Vladivostok and Chita. He said he believed in the idea of a Greater Russia, and that he had come to Ukraine to realize it. “Real Ukrainians have the right to live as they like,” he said. “They can create their own state which would be named Ukraine, or however they like, because the word Ukraine is a little humiliating,” he said, asserting that the literal translation meant “on the border of.” (The etymology is disputed.)