A historic trial over the deadly U.S. Capitol insurrection is drawing to a close, with prosecutors and defense lawyers set to make their final appeals to jurors before they decide the fate of Proud Boys extremist group leaders charged with plotting to use force to keep then-President Donald Trump in power.

A federal jury in Washington, D.C., is scheduled to begin hearing closing arguments on Monday after more than three months of testimony in the seditious conspiracy case against former Proud Boys national chairman Enrique Tarrio and four lieutenants.

Tarrio is one of the top targets of the Justice Department’s investigation of the riot that erupted at the Capitol on Jan. 6, 2021, as Congress prepared to certify Joe Biden's presidential election victory over Trump. Tarrio wasn’t in Washington, D.C., that day but is accused of orchestrating an attack from afar.

The Justice Department has already secured seditious conspiracy convictions against the founder and members of another far-right extremist group, the Oath Keepers. But this is the first major trial involving leaders of the far-right Proud Boys, a neofacist group of self-described "Western chauvinists" that remains a force in mainstream Republican circles.

Seditious conspiracy, a Civil War-era charge that is rare and can be difficult to prove, carries a potential sentence of up to 20 years in prison. The Proud Boys also face other serious charges.

PROUD BOYS' DEFENSE PLANS TO SUBPOENA TRUMP TO TESTIFY IN JAN. 6 SEDITIOUS CONSPIRACY TRIAL: REPORT

Jurors have heard 50 days of testimony by more than three dozen witnesses since the trial started in January. Two of the five defendants testified, but Tarrio wasn't one of them.

The foundation of the government's case is a trove of messages that Proud Boys leaders and members privately exchanged in encrypted chats — and publicly posted on social media — before, during and after the Jan. 6 attack.

The messages show Proud Boys celebrating when Trump, a Republican, told the group to "stand back and stand by" during his first debate with Biden, a Democrat. After the 2020 election, they discussed plans to travel to Washington for Trump's "Stop the Steal" rally on Jan. 6. And they raged online for weeks about baseless claims of a stolen election and what would happen when Biden took office.

"If Biden steals this election, (the Proud Boys) will be political prisoners," Tarrio posted on Nov. 16, 2020. "We won’t go quietly ... I promise."

Jurors also saw the string of gleeful messages that Proud Boys members posted during the Jan. 6 riot. A group of Proud Boys marched to the Capitol that day. Some entered the building after the mob of Trump supporters overwhelmed police lines.

"Make no mistake," Tarrio wrote in one message. "We did this."

Proud Boys leader Henry "Enrique" Tarrio

Proud Boys leader Henry "Enrique" Tarrio is pictured during a rally in Portland, Oregon, on Sept. 26, 2020. A federal jury is set to hear closing arguments in the trial of the Proud Boys, whose leaders were charged with plotting to use force to keep former President Donald Trump in power. (AP Photo/Allison Dinner, File)

Tarrio, a Miami resident, is on trial with Ethan Nordean, Joseph Biggs, Zachary Rehl and Dominic Pezzola. Nordean, of Auburn, Washington, was a Proud Boys chapter president. Biggs, of Ormond Beach, Florida, was a self-described Proud Boys organizer. Rehl was president of a Proud Boys chapter in Philadelphia. Pezzola was a Proud Boys member from Rochester, New York.

JAN. 6 SEDITIOUS CONSPIRACY TRIAL AGAINST PROUD BOYS BEGINS IN DOJ'S THIRD CASE BRINGING CIVIL WAR-ERA CHARGE 

Tarrio was arrested in Washington two days before the Jan. 6 riot on charges that he burned a church's Black Lives Matter banner during an earlier march in the city. Tarrio heeded a judge's order to leave the nation's capital after his arrest.

Defense attorneys have vigorously argued that there was no conspiracy and that the Proud Boys had no plan to attack the Capitol. And two key prosecution witnesses — onetime Proud Boys cooperating with the government — both acknowledged that they didn’t know of any such plan.

The defense attorneys called several current and former Proud Boys to the stand, trying to portray the group as a drinking club that only engaged in violence for self-defense against antifascist activists.

Rehl, the first defendant to testify, said the group had "no objective" that day. Pezzola testified that he got "caught up in the craziness" and acted alone on Jan. 6 when he used a riot shield stolen from a police officer to smash a Capitol window. Pezzola also referred to the proceedings as a "corrupt trial" with "fake charges."

Prosecutors have argued that Tarrio and the others mobilized a loyal group of foot soldiers — or "tools" — to supply the force necessary to carry out their plot.

Key witnesses for prosecutors included two former Proud Boys members who pleaded guilty to riot-related charges and are cooperating with the government in the hopes of getting lighter sentences.

The first, Matthew Greene, testified in January that group members were expecting a "civil war" as they grew increasingly angry about the election results. The second, Jeremy Bertino, testified in February that he viewed the Proud Boys as leaders of the conservative movement and as "the tip of the spear" after the November 2020 election.

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Approximately 20 Proud Boys leaders, members and associates have pleaded guilty to charges related to the Jan. 6 riot. Bertino is the only one to plead guilty to a seditious conspiracy charge.

The Proud Boys’ defense mirrored arguments made by lawyers for members of the Oath Keepers, who were separately charged with seditious conspiracy. They, too, said there was no evidence of a plan for group members to attack the Capitol.

Several Oath Keepers — including the antigovernment group’s founder, Stewart Rhodes — also took the witness stand in their trials, with mixed results. Over the course of two Oath Keepers trials, prosecutors secured seditious conspiracy convictions against Rhodes and five other members, while three defendants were acquitted of the charge. Those three, however, were convicted of obstructing Congress’ certification of Biden’s electoral victory.

Sentencings for Rhodes and other Oath Keepers are scheduled for next month. Prosecutors have yet to disclose how much prison time they will be seeking.