Even as the beleaguered police were still trying to disperse a violent mob at the Capitol last January, Stewart Rhodes, the leader of the far-right Oath Keepers militia, undertook a desperate, last-ditch effort to keep President Donald J. Trump in the White House, according to court papers released on Wednesday.

In a suite at the Phoenix Park Hotel, just blocks from the Capitol, Mr. Rhodes called an unnamed intermediary and, the papers said, repeatedly implored the person to ask Mr. Trump to mobilize his group to forcibly stop the transition of presidential power.

But the person refused to speak with Mr. Trump, the papers said. And once the call was over, Mr. Rhodes, turning to a group of his associates, declared, “I just want to fight.”

Witnessing this scene, which unfolded in the twilight hours of Jan. 6, 2021, was William Todd Wilson, a midlevel Oath Keepers leader from North Carolina. On Wednesday, Mr. Wilson, 44, pleaded guilty in federal court in Washington to charges of seditious conspiracy and agreed to cooperate with prosecutors in their investigation of the Oath Keepers’ role in the Capitol attack.

Mr. Wilson’s tale of what took place at the Phoenix Park — the same hotel that Enrique Tarrio, the leader of the far-right Proud Boys, had stayed at days earlier — was among the most dramatic accounts to have emerged so far in the government’s monthslong investigation of the Oath Keepers.

Phillip Linder, a lawyer for Mr. Rhodes, said he did not know who his client had called from the hotel in his effort to reach Mr. Trump.

In a 15-page statement of offense released in conjunction with his plea, Mr. Wilson also admitted to helping stockpile weapons in hotel rooms in Virginia for a so-called quick reaction force assembled to “provide firearms or cover to co-conspirators” who were “operating inside of Washington” on Jan. 6.

With his guilty plea, Mr. Wilson, a military and law enforcement veteran, became the third member of the Oath Keepers charged with sedition to reach a deal with the Justice Department to help in its most serious criminal case connected to the Capitol attack. As part of their inquiry, prosecutors have fanned out across the country interviewing dozens of members of the group. More than 20 Oath Keepers have been charged.

The new court papers paint a picture of Mr. Wilson as a man enraged by the results of the 2020 election. In early November, for example, he expressed outrage in an Oath Keepers group chat after Georgia was called for Joseph R. Biden Jr.

“Rigged,” he wrote. And then, “I’m ready to go coyote hunting.”

On Dec. 14, 2020 — the day that a majority of electors cast their votes for Mr. Biden in the Electoral College — Mr. Wilson saw an article posted in the group chat that was written by Michael T. Flynn, Mr. Trump’s onetime national security adviser. The article warned about “unelected tyrants,” and Mr. Wilson wrote to his compatriots, “It is time to fight.”

After several phone calls with Mr. Rhodes in early January, Mr. Wilson admitted driving from North Carolina to the Washington area on Jan. 5 with an AR-15-style rifle, a 9-milimeter pistol, 200 rounds of ammunition, body armor, pepper spray and a pocketknife. As he traveled, court papers say, he posted a message in the group chat, saying, “It’s going to hit the fan tonight!”

On the day of the attack, the papers said, Mr. Wilson, Mr. Rhodes and other Oath Keepers bypassed barricades at the Capitol, unlawfully entering a restricted area. As plumes of smoke rose from the ground, the papers said, Mr. Wilson heard Mr. Rhodes declare that they were in the middle of a “civil war.”

Moments later, the papers say, Mr. Wilson entered the Capitol armed with his pocketknife — the first Oath Keeper to have breached the building. He admitted that his goal in entering the building was to gather intelligence and to disrupt the final certification of the Electoral College count.

The sedition case against the Oath Keepers — one of two separate cases brought against members of the group — was made public in January with the arrest of Mr. Rhodes, a former Army paratrooper who went on to earn a law degree at Yale. In an indictment of Mr. Rhodes and 10 of his subordinates, prosecutors fleshed out a detailed portrait of a plot to disrupt the transfer of power from Mr. Trump to Mr. Biden, starting shortly after Election Day and continuing even after the Capitol was attacked.

Just two days after voting ended, prosecutors say, Mr. Rhodes told several members of his group to refuse to accept Mr. Biden’s victory — by force, if necessary.

“We aren’t getting through this without a civil war,” he wrote on the encrypted chat app Signal. “Too late for that. Prepare your mind, body, spirit.”

Throughout November and December, Mr. Rhodes issued an increasingly threatening — and paranoid — series of communiqués, calling on Mr. Trump to invoke the Insurrection Act and saying he had men stationed outside Washington ready to act on the president’s command. In the days leading up to the storming of the Capitol, Mr. Rhodes went on a gun-buying spree, spending thousands of dollars on military-grade firearms, ammunition and other tactical gear, prosecutors say.

While Mr. Rhodes never entered the Capitol, several members of the Oath Keepers did. Some have been accused of seeking to hunt down Speaker Nancy Pelosi. Others have been charged with assaulting police officers.

Through their lawyers, those facing charges have repeatedly said they converged on Washington just before Jan. 6 not to attack lawmakers, but instead as part of a security detail tasked with protecting conservative celebrities like Roger J. Stone Jr., a longtime ally of Mr. Trump.

According to the group’s internal communications, the Oath Keepers sometimes performed security work in the chaotic postelection period with another far-right paramilitary outfit, the 1st Amendment Praetorian.

By pleading guilty and agreeing to cooperate with prosecutors, Mr. Wilson appeared to put Mr. Rhodes in even more legal jeopardy.

He will most likely be able to help the government better understand the composition and mission of the quick reaction forces, which were stationed in Virginia and were said to have been poised to aid the Oath Keepers at the Capitol if Mr. Trump had invoked the Insurrection Act. According to the new court papers, Mr. Wilson also heard Mr. Rhodes discussing the need on multiple occasions to “engage in force, up to and including lethal violence, in order to stop the transfer of power.”

Mr. Wilson joins two other Oath Keepers charged with sedition — Brian Ulrich and Joshua James — in reaching cooperation deals with the government. In the past month or so, prosecutors have also struck similar arrangements with three key members of the Proud Boys, which also played a crucial role in the Capitol attack.

When Mr. Ulrich, 44, pleaded guilty last week, he admitted to rushing to the Capitol with five compatriots in golf carts then marching into the building while the police were trying to clear it. In the days leading up to the attack, he also acknowledged sending messages in a private Oath Keepers group chat, saying that “civil war” would be necessary if Mr. Biden took office.

“Trump acts now a few hundred radicals die trying to burn down cities,” Mr. Ulrich wrote in the chat on Dec. 19, 2020 — the same day Mr. Trump posted a tweet urging his supporters to go to Washington for a “wild” protest. “Trump sits on his hands Biden wins … millions die resisting the death of the 1st and 2nd amendment.”

During his own guilty plea in March, Mr. James, the leader of an Alabama Oath Keepers chapter, said he had gone with Mr. Ulrich to the Capitol in a golf cart and assaulted a police officer in the building. After the riot, Mr. James acknowledged helping Mr. Rhodes get out of Washington by taking some of the arms and ammunition that the Oath Keepers leader had stored in his vehicle.

Mr. James, 33, has admitted to being involved in meetings with Mr. Rhodes within weeks of the election where he learned about the Oath Keepers’ “plans to oppose by force the lawful transfer of presidential power,” court papers say. He has also acknowledged helping Mr. Rhodes arrange a conference call on the online meeting site GoToMeeting to “facilitate planning” for Jan. 6.

While Mr. James served on Mr. Stone’s protective detail before the Capitol was stormed, it remains unclear if he has provided prosecutors with any information about the longtime Trump adviser. According to private group chats seized by the government, the leader of the Oath Keepers’ Florida chapter, Kelly Meggs, reached a deal to protect Mr. Stone in early January.