First Jan. 6 Trial, of Guy Wesley Reffitt, Starts Monday

When the first trial stemming from the attack on the Capitol opens on Monday, it will set the stage for prosecutors to do more than merely lay out the details of how the defendant, Guy Wesley Reffitt, sought to storm the building with a pistol at his hip.

For the first time in a courtroom, they will present a broad portrait of the violent chaos that erupted that day and seek to persuade a jury that the pro-Trump mob that Mr. Reffitt is accused of joining struck at the heart of American democracy by disrupting the transition of presidential power.

The trial, which will take place in Federal District Court in Washington and begin on Monday morning with jury selection, may not be the flashiest or most significant of the dozens of Capitol riot cases that are scheduled to go to trial this year.

But because it is the first to reach a courtroom, it will most likely set the tone for those that follow and serve as a kind of proving ground for the charges prosecutors have filed against hundreds of defendants. (More than 200 people have already pleaded guilty in cases related to the Capitol attack.)

At the heart of Mr. Reffitt’s case is the accusation that the defendant, an oil industry worker with purported ties to a Texas militia, obstructed the work of Congress on Jan. 6, 2021, when a joint session of the House and the Senate met to certify the results of the 2020 election. Prosecutors say he donned body armor and a helmet mounted with a video camera, and placed himself at “the front of the pack” that charged the Capitol.

The obstruction charge — one of five counts lodged against him — has been used in lieu of other crimes like sedition or insurrection in scores of Capitol riot cases to describe the disruption that occurred when the mob forced lawmakers to flee.

Passed in 2002 as part of the Sarbanes-Oxley Act, which sought to reduce corporate malfeasance, the obstruction provision was initially intended to prohibit activities like shredding documents or tampering with witnesses in congressional inquiries.

Several defense lawyers — among them Mr. Reffitt’s — tried to get the charge dismissed, arguing that prosecutors had stretched it far beyond its scope and were trying to criminalize behavior that too closely resembled ordinary protest protected by the First Amendment.

But 10 federal judges — including Dabney L. Friedrich, who is overseeing Mr. Reffitt’s case — disagreed and allowed the charge to be used. The Reffitt trial will be the first time a jury will get to decide whether the charge fits the crime, though in an unusual move, Judge Friedrich has said she will strike the count before it goes to the jury if the government fails to prove its case.

Beyond such technical matters, the trial will feature what amounts to a panoramic view of the violence at the Capitol.

Prosecutors have said, for example, that they intend to prove their claim that a “civil disorder” occurred that day by showing the jury “the progression of the riot” through a compilation of surveillance videos from both outside and inside the building. The jury is also expected to see portions of a 31-minute video that Mr. Reffitt recorded during the riot as well as a clip of Vice President Mike Pence hurrying down a staircase after attackers breached the building.

Three Capitol Police officers are set to testify about their firsthand attempts to stop Mr. Reffitt — and others — from breaking past security by firing pepper balls and spraying tear gas at the crowd. One of those officers, Shauni Kerkhoff, is scheduled to be the first witness at the trial, prosecutors have said.

Mr. Reffitt’s lawyer, William L. Welch III, has not said much about the defense he intends to mount, but his client has openly pushed back against accusations that he took part in anything untoward at the Capitol. In a letter obtained by ProPublica last spring, Mr. Reffitt wrote of the attack, “There was no insurrection, no conspiracy, no sinister plan and no reason to think otherwise.”

On Thursday, a message purportedly written by Mr. Reffitt was posted on the “J6 Patriot News” channel on Telegram, discussing the “beginning of the 1/6 Political Prisoner Trials.”

“I am prepared to stare down the barrel of tyranny to receive the bullet of freedom,” the message said.

The case is one of several that will feature evidence about the U.S. militia movement. Prosecutors say that Mr. Reffitt was a member of the Texas Three Percenters, one of several state chapters of a larger organization that focuses on gun rights and antigovernment activity.

While the Three Percenters may not be as prominent as other far-right groups like the Proud Boys or the Oath Keepers, several purported members of the movement have been charged in connection with the Capitol attack, including at least four from Southern California.

To prove Mr. Reffitt’s militia ties, prosecutors are set to call as a witness a fellow Three Percenter who plans to testify under an immunity deal with the government. They say the witness — known for now only by the initials R.H. — intends to tell the jury about the arrangements he made with Mr. Reffitt to travel to Washington and the firearms and tactical gear, including an assault rifle, that Mr. Reffitt brought on the trip.

Two members of Mr. Reffitt’s family — his son and daughter — are also expected to take the stand against him.

Prosecutors say the son, who was 18 at the time of the attack, will tell the jury that he and his sister, then 16, spoke with their father when he returned from Washington after the riot and that Mr. Reffitt threatened to shoot them if they went to the F.B.I. about him.

In a previous court hearing, the daughter testified that Mr. Reffitt threatened to put a bullet in her cellphone if she posted about him on social media.