WASHINGTON — On the afternoon of Jan. 6, 2021, a man believed by the authorities to be a member of a Texas militia walked from the Ellipse in Washington, where President Donald J. Trump had been speaking about election fraud, to the Capitol. He was wearing what he described as his “full battle rattle”: body armor, a helmet fixed with a camera, a set of plastic hand ties and a .40-caliber pistol at his hip.

Over the next several minutes, prosecutors said, the armed man, Guy Wesley Reffitt, not only helped lead a mob up a staircase of the building, but also recorded himself narrating his role in the advance.

“We’re taking the Capitol before the day is over, ripping them out by their hair,” Mr. Reffitt said on camera. Then he made a specific threat against the Speaker of the House: “I just want to see Nancy Pelosi’s head hitting every step on the way out.”

With a replay of this dramatic scene, prosecutors on Wednesday opened the first criminal trial stemming from the Capitol attack, saying that Mr. Reffitt was at the forefront of the pro-Trump crowd that stormed into the building as lawmakers were certifying the results of the 2020 election.

“He planned to light the match that would start the fire,” Jeffrey S. Nestler, a federal prosecutor, said in an opening statement. “He wanted to stop Congress from doing its job.”

As the trial unfolds over the next several days, the government intends to offer evidence that Mr. Reffitt, 41, had a standoff with the police outside the Capitol after traveling to Washington from his home in Wylie, Texas. Prosecutors say Mr. Reffitt came to Washington with a fellow member of the Texas Three Percenters, a loosely organized militia movement that takes its name from the supposed 3 percent of the U.S. colonial population that fought against the British.

Mr. Nestler said that prosecutors would introduce messages that Mr. Reffitt had sent to other members of the group in advance of the attack, saying that “the fuel is set” and that he planned to “strike the match in DC on Jan. 6.” The Three Percenter who traveled with him to Washington, Rocky Hardy, is set to testify under an immunity deal with the government.

Prosecutors also plan to elicit testimony from two of Mr. Reffitt’s children, Jackson and Peyton, who were teenagers at the time of the attack. The children plan to say their father threatened them after he returned to Texas in order to keep them from turning him into the authorities.

A large man with a barrel chest and a pigtail, Mr. Reffitt sat without showing much emotion as Mr. Nestler told the jury how he had led a large group of rioters up a staircase at the Capitol, just outside the Senate chamber, brushing off attempts by the police to stop him with pepper balls and heavier projectiles. Even after he was finally subdued with a canister of pepper spray, Mr. Reffitt urged the crowd around him to push on, Mr. Nestler said.

In his own brief opening statement, Mr. Reffitt’s lawyer, William L. Welch, played down the confrontation with police, saying that his client had never assaulted any officers or helped anyone assault them. While Mr. Welch admitted that Mr. Reffitt was prone to “rants” and “hyperbole,” he insisted that he was not aggressive.

“This case has been a rush to judgment,” Mr. Welch said, “based on bragging, based on a lot of hype.”

The first witness in the trial, Shauni Kerkhoff, a former Capitol Police officer, appeared to contradict Mr. Welch’s description of Mr. Reffitt. While watching a surveillance camera video of her standoff with Mr. Reffitt, Ms. Kerkhoff told the jury that after he moved toward her up the staircase, disobeying her commands, she used a Tippmann PepperBall Launcher to fire 40 to 50 projectiles.

That did little to slow him down, she recalled. Mr. Reffitt was also undeterred by larger projectiles fired by her partner and by pepper spray used by a third officer.

At that point, Ms. Kerkhoff made a panicked call for help on her radio.

“We have an individual breaching the west terrace, breaching the west terrace up the stairs,” she called out in a breathless message played for the jury. “We need backup.”

After more than 200 guilty pleas in Jan. 6-related cases, Mr. Reffitt’s is the first to reach a courtroom and marks the first time a jury will consider the legal theory under which the government has charged hundreds of defendants.

At the heart of the case is the accusation that Mr. Reffitt obstructed the work of Congress during the attack by helping chase lawmakers from the building while they were overseeing the certification of the Electoral College vote. 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.

In the months leading up to the trial, several defense lawyers — including Mr. Reffitt’s — challenged the use of the law, arguing that prosecutors had stretched it far beyond its original design as a way to curb activities like shredding documents or tampering with witnesses in congressional inquiries. But 10 federal judges — including Judge Dabney L. Friedrich, who is overseeing the Reffitt trial — have upheld the use of the law. Mr. Reffitt’s case will be the first time a jury will get to decide whether the charge fits the crime.

There is one wrinkle, however: Judge Friedrich ruled before the trial that she might toss the obstruction charge on her own if she does not believe the evidence supports the claim that Mr. Reffitt acted “corruptly” in disrupting Congress’s work, as required by the statute. Prosecutors will seek to convince her — and the jury — that Mr. Reffitt did act corruptly by claiming that he meant to assault the police and, perhaps, through how he planned for violence that day.

To that end, Mr. Nestler told the jury in his opening statement that Mr. Reffitt had started to prepare for the storming of the Capitol well before Jan. 6. In late December, he sent a message to members of his family saying that “tyranny” was coming and that “what’s about to happen will shock the world.”

Mr. Reffitt’s son Jackson was sufficiently concerned that he called the F.B.I. on Christmas Eve 2020 to warn agents about his father — “a wrenching decision,” as Mr. Nestler put it.

Less than two weeks later, Mr. Reffitt and Mr. Hardy set out from Texas and drove 2,000 miles across the country to Washington. They took a car, Mr. Nestler, said, so that they could bring a cache of weapons — among them, the pistol Mr. Reffitt wore and an assault rifle.