Stewart Rhodes, the leader of the Oath Keepers militia charged with seditious conspiracy in connection with the attack on the Capitol on Jan. 6 last year, was denied bail on Wednesday by a federal judge in Texas who said he was a flight risk partly because of the “elaborate escape tunnels” he had installed in his backyard.

Mr. Rhodes, 56, lived in fear of being “picked up by the feds” and bought hundreds of thousands of dollars of razor wire intended for the perimeter of his property in Montana, Judge Kimberly C. Priest Johnson wrote in a 17-page order. Mr. Rhodes, Judge Johnson said, also stashed “unregistered cars in the woods” near his home.

The charges against Mr. Rhodes, who was accused of sedition this month with 10 other members of his group, are part of the most serious criminal case the Justice Department has brought in connection with the Capitol attack. This week, a federal judge in Washington who will oversee the case set a tentative trial for July.

Prosecutors have accused about 275 people of obstructing Congress’s duty to certify the 2020 presidential vote. But they had not previously used a sedition charge, with the legal weight and political overtones it carries in a highly polarized country.

Beginning only days after the 2020 election, prosecutors say, Mr. Rhodes oversaw a complex plot “to oppose the lawful transfer of presidential power by force.”

Some members of the Oath Keepers — a group made up largely of current and former law enforcement officers and members of the military — broke into the Capitol in a military-style formation on Jan. 6, 2021, and went in search of Speaker Nancy Pelosi, the indictment said. Others, it said, were stationed in a hotel in Arlington, Va., as an armed “quick reaction force,” ready to rush into Washington if needed.

Some of the quick reaction force’s weapons came from Mr. Rhodes himself, who bought nearly $40,000 of firearms, ammunition and related accessories in the days leading up to the attack, Judge Johnson wrote. In private communication obtained by the government through search warrants, Mr. Rhodes spoke often about “inciting a revolution or civil war” that “had the potential to be massively bloody,” she added.

Mr. Rhodes’s lawyer said he planned to appeal the judge’s decision.

More than 20 members of the Oath Keepers have been charged in connection with the Capitol attack, including at least four who are known to be cooperating with federal prosecutors. Through their lawyers, the Oath Keepers who are facing charges have said they had converged on Washington just before Jan. 6 not to attack the Capitol, but as part of a security detail hired to protect conservative celebrities like Roger J. Stone Jr., a longtime ally of former President Donald J. Trump.

In an unusual turn of events, Mr. Rhodes’s estranged former wife, Tasha Adams, reached out to Judge Johnson after his bail hearing on Monday, asking for her permission to offer information about their marriage. After noting that she had filed for divorce in 2018, Ms. Adams told the judge that Mr. Rhodes often brandished weapons in their home “to control her behavior” and that he physically abused their six children “under the guise of participating in ‘martial arts practice.’”

“Ms. Adams testified that defendant’s violence toward the family became more frequent in 2016 and that her greatest fear was that defendant would murder Ms. Adams and the children before committing suicide,” Judge Johnson wrote.

Marina Trahan Martinez contributed reporting.