WASHINGTON — The House committee investigating the Jan. 6 attack at the Capitol issued a subpoena on Thursday for Phil Waldron, a retired Army colonel with a background in information warfare who had circulated a detailed and extreme plan to overturn the 2020 election.

The committee has been scrutinizing Mr. Waldron’s role in spreading false information about the election since a 38-page PowerPoint presentation he circulated on Capitol Hill was turned over to the panel by Mark Meadows, President Donald J. Trump’s last chief of staff, who denied having anything to do with it.

“The document he reportedly provided to administration officials and members of Congress is an alarming blueprint for overturning a nationwide election,” Representative Bennie Thompson, Democrat of Mississippi and chairman of the committee, said.

The PowerPoint — titled “Election Fraud, Foreign Interference & Options for 6 JAN” — recommended that Mr. Trump declare a national emergency to cling to power and included the false claim that China and Venezuela had obtained control over the voting infrastructure in a majority of states.

On Jan. 4, associates of Mr. Waldron spoke to a group of senators and informed them about the allegations of election fraud in the PowerPoint, Mr. Waldron told The New York Times recently in an interview. On Jan. 5, he said, he personally briefed a small group of House members whom he did not identify; that discussion also focused on baseless claims of foreign interference in the election. He said he had made the document available to the lawmakers.

Mr. Waldron told The Washington Post that he had contributed to the creation of the document and had visited the White House several times after last year’s election, and spoken with Mr. Meadows “maybe eight to 10 times.”

Mr. Waldron, who specialized in psychological influence operations and once was deployed to Iraq, retired from the military in 2016 after 30 years of service. He appears to lead a quieter life these days, describing himself on his LinkedIn page as the founder, forklift driver and floor sweeper at One Shot Distillery and Brewery in Dripping Springs, Texas.

But almost as soon as the 2020 polls closed, he joined a wide-ranging effort to persuade the public and key Republican politicians that the vote count had been marred by rampant fraud.

By mid-November, Mr. Waldron was in contact with Rudolph W. Giuliani, Mr. Trump’s personal lawyer, who at the time was overseeing challenges to the election. Mr. Waldron fed Mr. Giuliani information about alleged attempts by foreign powers to hack American voting machines and about suspected left-wing operatives who were working for the vote tabulation company Dominion Voting Systems. Some of these baseless claims ultimately made their way into federal lawsuits attacking Dominion’s role in the election that were filed by the pro-Trump lawyer Sidney Powell.

“Colonel in the military, great war record,” Mr. Giuliani later said of Mr. Waldron in a deposition he gave in a defamation lawsuit brought by a Dominion employee. “I’ve had substantial dealings with him and he’s very, very thorough and very experienced in this kind of work.”

Mr. Giuliani said that Mr. Waldron’s legal team put up a “big whiteboard” that laid out its strategies while he and fellow lawyers, including Ms. Powell and Jenna Ellis, ran operations as “really active supervisors.”

Mr. Giuliani said another lawyer, Boris Epshteyn, was focusing on fraud allegations in Nevada and Arizona, while Mr. Waldron was investigating conspiracies related to Dominion voting machines.

“If I were to think of Dominion, I would think of Sidney carrying the ball on that, with everybody else helping, and Phil was the investigator,” Mr. Giuliani said.

Mr. Waldron also participated in planning meetings at the Willard Hotel in Washington, D.C. in early January concerning efforts to challenge the election results, according to the committee.

In the wake of the election, Mr. Waldron was working closely with a Texas-based company called Allied Security Operations Group, whose co-founder, Russell J. Ramsland Jr., was also helping Ms. Powell with her lawsuits by sending her affidavits claiming that Dominion’s voting machines could be easily hacked by foreign powers. As early as August 2020, even before a single vote was cast, according to court papers filed by Dominion on Tuesday, Mr. Ramsland had been hired by Patrick Byrne, the former chief executive of Overstock.com, to “reverse engineer” the evidence needed to “mislead people into believing” that Dominion helped steal the 2020 election.

But the legal campaign against Dominion was only one part of what amounted to a full-court press against the validity of the vote results. Mr. Ramsland also began to appear on right-wing media outlets like Fox News and Newsmax to promote conspiracy theories about the election. He claimed, for example, that computer servers based in Germany had been used to flip votes in the United States. Mr. Trump amplified that allegation on Twitter.

Not long after, Allied Security became involved in writing a report about election results in Antrim County, Mich., that falsely claimed voting machines there had a 68 percent error rate. That allegation too ended up on Mr. Trump’s Twitter feed.

When legal challenges against the election failed, Mr. Waldron took a new tack, joining Mr. Giuliani at unofficial “election fraud” hearings conducted by state lawmakers in a handful of swing states. The purpose of the hearings was not only to further promote Mr. Trump’s falsehoods about the election but also to seed the ground for state legislators to exercise control over the election results.

In late November, Mr. Waldron appeared at a hearing conducted by the Pennsylvania Senate and offered up the baseless claim that more than a million votes in the state could have been “altered or fraudulent.” To fix this supposed problem, he called for a “detailed forensic analysis” of the voting machines and software used in Pennsylvania.

At the end of the hearing, a surprise guest called in by speaker phone: Mr. Trump. He told attendees that he had been watching the proceeding from the Oval Office on OAN, the far-right television channel, and found it “very interesting.”

Days later, Mr. Waldron appeared with Mr. Giuliani at another hearing in Phoenix and told the crowd that Dominion’s voting machines were vulnerable to manipulation. “Your vote is not as secure as your Venmo account,” he said. A video clip of his testimony was later posted on Mr. Trump’s official YouTube page.

Mr. Waldron is still at it. On Tuesday, he appeared at a hearing of the Louisiana State Legislature to deliver a presentation about voting machines and election systems that he billed as “Built for Integrity.” He struck themes that have formed the basis of his complaints about the 2020 election, railing against mail-in ballots and signature verifications and suggesting that state lawmakers had the ultimate authority to decide election results.