A federal judge on Wednesday sentenced a rioter who savagely assaulted an officer defending the Capitol on Jan. 6, 2021, to more than 12 years in prison, calling him a “one-man army of hate” whose severe punishment might act as a deterrent to future acts of political violence.

The 151-month sentence, handed down at a two-and-a-half-hour hearing in Federal District Court in Washington, was one of the stiffest so far in the Justice Department’s sprawling investigation of the Capitol attack. It stemmed from one of the most wrenching episodes of the day, an assault on a District of Columbia police officer with a Taser-like weapon that left him unconscious and unable to return to his duties.

The defendant, Daniel Rodriguez, 40, who had previously admitted to driving from California to Washington to do armed battle on behalf of former President Donald J. Trump, expressed some regret for his actions as he asked the judge for leniency. But after receiving his sentence, Mr. Rodriguez smiled and let out a defiant shout of “Trump won!” before being led out of the room by federal marshals.

The judge, Amy Berman Jackson, rejected defense arguments that Mr. Rodriguez was the product of a difficult upbringing and that he had been a mostly law-abiding retail and warehouse worker before he became radicalized by what she called Mr. Trump’s “irresponsible and knowingly false claims that the election was stolen.”

Judge Jackson, her voice rising with disgust as she documented his actions in detail, said she was sympathetic to Mr. Rodriguez’s claim that his extended absence has been harmful to his ailing mother, but she cast the stiff sentence as serving a higher purpose of safeguarding democracy from the continuing threats.

“The shadow of tyranny has not gone away,” said Judge Jackson, who was appointed by President Barack Obama.

Patriotism, she told Mr. Rodriguez, “is loyalty to your country, not to a single head of state.”

Few among the more than 1,000 people who have been charged in connection with the Capitol attack were as violent as Mr. Rodriguez, a single, fatherless man who, according to his lawyer, “idolized” Mr. Trump and his MAGA movement.

Over the course of nearly two hours at the Capitol on Jan. 6, prosecutors say, Mr. Rodriguez sprayed a fire extinguisher at the police, shoved at officers with a wooden pole, took part in a “heave ho” effort to break police lines and eventually assaulted Officer Michael Fanone of the Metropolitan Police Department in Washington — who had rushed to the scene when he heard law enforcement calls for help — by hitting him twice in the neck with an electroshock device in a crowd outside the building.

Even then, prosecutors say, Mr. Rodriguez kept going. He entered the Capitol and sought to rile up other rioters, they said, and tried to smash a window with a pole-like object he found inside. He also ransacked offices, the government says, and instructed others in the mob to go through drawers to “look for intel.”

When Mr. Rodriguez finally left the Capitol grounds, prosecutors say, he sent a text message to a group chat he had created called Patriots 45 MAGA Gang, showing a gallows with the Capitol in the background. The text of the message read, “No Democrats found unfortunately.”

“These people are zealots,” Mr. Fanone, who attended the hearing, said afterward. “They need to be held accountable.”

In court papers filed before the sentencing, Mr. Rodriguez’s lawyers wrote that their client was one of millions duped by the former president, who “doubled down on his lies and falsely declared that he had won.”

Mr. Rodriguez, who grew up without a father and never completed high school, was one of those people. He “deeply respected and idolized Trump,” the lawyers wrote, adding, “He saw the former president as the father he wished he had.”

But Mr. Rodriguez did little to help his own cause in the courtroom. He veered from that script during a rambling 25-minute statement in which he seemed to cast the period leading up to the attack on the Capitol in nostalgic terms — a time when he bonded with fellow Trump supporters, practiced military drills by playing paintball and smoked marijuana, in his retelling.

“I did what I thought was right at the time,” he said.

Judge Jackson, in passing sentence, was not swayed. She said she was particularly confounded by one thing Mr. Rodriguez had just said — that he had armed himself in anticipation of a fight with law enforcement, to participate in a demonstration intended to safeguard the police under a “Blue Lives Matter” banner.

“Today was not the best day to say you had to be armed and ready because police don’t always do the right thing,” she said, as one of his lawyers slumped in her seat.

Prosecutors say that Mr. Rodriguez set up Patriots 45 MAGA Gang on Telegram in the fall of 2020. But after the election — and Mr. Trump’s repeated lies about the outcome being marred by fraud — the group chat became “a breeding ground” for “plans for violence against the seat of the federal government.”

Those plans crystallized, prosecutors said, after Mr. Trump posted a message on Twitter on Dec. 19, 2020, summoning his supporters to Washington for a “wild” protest on Jan. 6.

After the Tweet was posted, Mr. Rodriguez urged others in the chat to rent an R.V. and drive to Washington instead of flying so that they could bring weapons. He also encouraged chat members, prosecutors said, to arm themselves with knives, bear spray, even ax handles.

“There will be blood,” he wrote in the chat on the night before the Capitol attack. “Welcome to the revolution.”

Edward Badalian, Mr. Rodriguez’s co-defendant and fellow group chat member, was convicted of conspiracy and obstruction of an official proceeding in April after a bench trial in front of Judge Jackson. Mr. Rodriguez pleaded guilty to similar charges and assault in February.

Mr. Fanone, who has left the police force and was conspicuous in the courtroom in his cowboy boots and neck tattoos, watched impassively as the prosecution played video from his body camera showing him losing consciousness, being dragged to safety and asking, faintly, if officers had repelled the attackers.

But he could not sit still when Mr. Rodriguez began his long statement, and bolted from the courtroom around the time the defendant described his fellow attackers as “a group of misfits” who bonded over “smoking weed” together.

“We are all a lot dumber for just having lived through that,” Mr. Fanone said in the corridor.