SPOKANE, Wash. — Matt Shea was 34 years old when he ran for the State Legislature in eastern Washington, but he had already established credentials that made him a promising Republican candidate.

A lawyer trained at Gonzaga University who had served a tour in Iraq with Washington’s Army National Guard, Mr. Shea pitched voters in 2008 on a platform of limiting taxes and punishing criminals, opposing same-sex marriage and supporting gun rights. He went on to win with nearly 60 percent of the vote, then moved up the ranks in the Legislature, reaching the powerful position of chair of his party’s caucus in 2017.

But back in his home district, Mr. Shea also began attracting the attention of law enforcement for his growing embrace of fringe ideologies and conspiracy theories. He networked with local militia groups, talked about plans to create a 51st state called Liberty and distributed to his closest followers a “Biblical Basis for War” document that calls for the “surrender” of those who favor abortion rights, same-sex marriage, “idolatry” and communism. “If they do not yield — kill all males,” it said.

Last week, a report commissioned by the State Legislature asserted that Mr. Shea had engaged in domestic terrorism in his support of the armed takeover of Oregon’s Malheur National Wildlife Refuge by militant ranchers and their supporters in 2016 — part of a protest over federal ownership of public lands in the West.