WASHINGTON — The Oath Keepers founder, Stewart Rhodes, was convicted Tuesday of seditious conspiracy for a violent plot to overturn Democrat Joe Biden’s presidential election win, handing the Department of Justice a major victory in its widespread prosecution of the January 6, 2021, insurrection.

A District of Columbia jury found Rhodes guilty of sedition after three days of deliberations in the nearly two-month-long trial that showcased the far-right extremist group’s efforts to keep President Trump in the White House at all costs.

Using dozens of encrypted messages, recordings, and surveillance video, prosecutors made the case that Rhodes began shortly after the 2020 election to prepare an armed rebellion to stop the transfer of presidential power.

During seven weeks of testimony, jurors heard how Rhodes rallied his followers to fight to defend Mr. Trump, discussed the prospect of a “bloody” civil war, and warned the Oath Keepers may have to “rise up in insurrection” to defeat Mr. Biden if Mr. Trump didn’t act.

Rhodes and a co-defendant who was also convicted of seditious conspiracy are the first people in nearly three decades to be found guilty of the rarely used Civil War-era charge at trial. The trial was the biggest test yet for the justice department in its efforts to hold accountable those responsible for attack that shook the foundations of American democracy.

Seditious conspiracy calls for up to 20 years behind bars.

On trial alongside Rhodes, of Granbury, Texas, were the leader of the Florida chapter of the Oath Keepers, Kelly Meggs; another Florida Oath Keeper, Kenneth Harrelson; a retired Navy intelligence officer from Virginia, Thomas Caldwell; and an Ohio militia group leader, Jessica Watkins.

Defense attorneys accused prosecutors of twisting their clients’ words and insisted the Oath Keepers came to Washington only to provide security for figures such as Roger Stone, a longtime ally of Mr. Trump. The defense focused heavily on seeking to show that Rhodes’s rhetoric was just bluster and that the Oath Keepers had no plan before January 6 to attack the Capitol.

Rhodes testified that he had no idea that his followers were going to join the mob and storm the Capitol and said he was upset after he found out that some did. Rhodes said they were acting “stupid” and outside their mission for the day.

Prosecutors said the Oath Keepers saw an opportunity to advance their plot to stop the transfer of power and sprang into action when the mob started storming the Capitol. The Capitol attack was a “means to an end” for the Oath Keepers, an assistant United States attorney, Kathryn Rakoczy, told jurors in her closing argument.

Jurors heard how Rhodes spent thousands of dollars on an AR-platform rifle, magazines, mounts, sights, and other equipment on his way to Washington ahead of the riot. They watched surveillance footage from the Virginia hotel where some Oath Keepers stashed weapons for “quick reaction force” teams prosecutors said were ready to get weapons into the city quickly if they were needed. The weapons were never deployed.

On January 6, Oath Keepers wearing combat gear were seen on camera shouldering their way through the crowd and into the Capitol. Rhodes remained outside like a “general surveying his troops on the battlefield,” a prosecutor said. After the riot, Rhodes and other Oath Keepers went to an Olive Garden restaurant to celebrate, according to prosecutors.

The trial disclosed new details about Rhodes’s efforts to pressure Mr. Trump to fight to stay in White House in the weeks leading up to January 6. Shortly after the election, in a group chat that included Stone called “FOS,” or “Friends of Stone,” Rhodes wrote, “So will you step up and push Trump to FINALLY take decisive action?”

Another man testified that after the riot, Rhodes tried to persuade him to pass along a message to Mr. Trump that urged the president not to give up his fight to hold onto power. The intermediary — a man who told jurors he had an indirect way to reach the president — recorded his meeting with Rhodes and went to the FBI instead of giving the message to Mr. Trump.

“If he’s not going to do the right thing and he’s just gonna let himself be removed illegally then we should have brought rifles,” Rhodes said during that meeting, according to a recording played for jurors. “We should have fixed it right then and there. I’d hang [expletive] Pelosi from the lamppost,” Rhodes said, referring to House speaker, Nancy Pelosi.

Three other Oath Keepers previously pleaded guilty to seditious conspiracy. The last time the justice department had secured such a conviction at trial, though, was in the 1995 prosecution of Islamic militants who plotted to bomb New York City landmarks.