A lawyer allied with President Donald J. Trump first laid out a plot to use false slates of electors to subvert the 2020 election in a previously unknown internal campaign memo that prosecutors are portraying as a crucial link in how the Trump team’s efforts evolved into a criminal conspiracy.

The existence of the Dec. 6, 2020, memo came to light in last week’s indictment of Mr. Trump, though its details remained unclear. But a copy obtained by The New York Times shows for the first time that the lawyer, Kenneth Chesebro, acknowledged from the start that he was proposing “a bold, controversial strategy” that the Supreme Court “likely” would reject in the end.

But even if the plan did not ultimately pass legal muster at the highest level, Mr. Chesebro argued that it would achieve two goals. It would focus attention on claims of voter fraud and “buy the Trump campaign more time to win litigation that would deprive Biden of electoral votes and/or add to Trump’s column.”

The memo had been a missing piece in the public record of how Mr. Trump’s allies developed their strategy to overturn Mr. Biden’s victory. In mid-December, the false Trump electors could go through the motions of voting as if they had the authority to do so. Then, on Jan. 6, 2021, Vice President Mike Pence could unilaterally count those slates of votes, rather than the official and certified ones for Joseph R. Biden Jr.

While that basic plan itself was already known, the document, described by prosecutors as the “fraudulent elector memo,” provides new details about how it originated and was discussed behind the scenes. Among those details is Mr. Chesebro’s proposed “messaging” strategy to explain why pro-Trump electors were meeting in states where Mr. Biden was declared the winner. The campaign would present that step as “a routine measure that is necessary to ensure” that the correct electoral slate could be counted by Congress if courts or legislatures later concluded that Mr. Trump had actually won the states.

It was not the first time Mr. Chesebro had raised the notion of creating alternate electors. In November, he had suggested doing so in Wisconsin, although for a different reason: to safeguard Mr. Trump’s rights in case he later won a court battle and was declared that state’s certified winner by Jan. 6, as had happened with Hawaii in 1960.

But the indictment portrayed the Dec. 6 memo as a “sharp departure” from that proposal, becoming what prosecutors say was a criminal plot to engineer “a fake controversy that would derail the proper certification of Biden as president-elect.”

“I recognize that what I suggest is a bold, controversial strategy, and that there are many reasons why it might not end up being executed on Jan. 6,” Mr. Chesebro wrote. “But as long as it is one possible option, to preserve it as a possibility it is important that the Trump-Pence electors cast their electoral votes on Dec. 14.”

Three days later, Mr. Chesebro drew up specific instructions to create fraudulent electors in multiple states — in another memo whose existence, along with the one in November, was first reported by The Times last year. The House committee investigating the Jan. 6 riot also cited them in its December report, but it apparently did not learn of the Dec. 6 memo.

“I believe that what can be achieved on Jan. 6 is not simply to keep Biden below 270 electoral votes,” Mr. Chesebro wrote in the newly disclosed memo. “It seems feasible that the vote count can be conducted so that at no point will Trump be behind in the electoral vote count unless and until Biden can obtain a favorable decision from the Supreme Court upholding the Electoral Count Act as constitutional, or otherwise recognizing the power of Congress (and not the president of the Senate) to count the votes.”

Mr. Chesebro and his lawyer did not respond to requests for comment. A Trump spokesman did not respond to an email seeking comment.

The false electors scheme was perhaps the most sprawling of Mr. Trump’s various efforts to overturn the results of the 2020 election. It involved lawyers working on his campaign’s behalf across seven states, dozens of electors willing to claim that Mr. Trump — not Mr. Biden — had won their states, and open resistance from some of those potential electors that the plan could be illegal or even “appear treasonous.” In the end, it became the cornerstone of the indictment against Mr. Trump.

While another lawyer — John Eastman, described as Co-Conspirator 2 in the indictment — became a key figure who championed the plan and worked more directly with Mr. Trump on it, Mr. Chesebro was an architect of it. He was first enlisted by the Trump campaign in Wisconsin to help with a legal challenge to the results there.

Prosecutors are still hearing evidence related to the investigation, even after charges were leveled against Mr. Trump, according to people familiar with the matter. The House committee last year released emails its investigators obtained showing that Mr. Chesebro had sent copies of the two previously reported memos, one from Nov. 18 and another from Dec. 9, to allies in the states working on the fake electors plan.

But he did not attach his Dec. 6 memo to those messages, which laid out a more audacious idea: having Mr. Pence take “the position that it is his constitutional power and duty, alone, as president of the Senate, to both open and count the votes.” That is, he could resolve the dispute over which slate was valid by counting the alternate electors for Mr. Trump even if Mr. Biden remained the certified winner of their states.

Mr. Chesebro, who is described as Co-Conspirator 5 in the indictment but has not been charged by the special counsel, addressed the second memo to James R. Troupis, a lawyer who was assisting the Trump campaign’s efforts to challenge Mr. Biden’s victory in Wisconsin.

By the next day, the indictment said, Mr. Chesebro’s memo had reached Rudolph W. Giuliani, Mr. Trump’s personal lawyer.

According to the indictment, Mr. Giuliani, who is referred to as Co-Conspirator 1, spoke with someone identified only as Co-Conspirator 6 about finding lawyers to help with the effort in seven states. An email reviewed by The Times suggests that particular conspirator could be Boris Epshteyn, a campaign strategic adviser for the Trump campaign who was paid for political consulting. That day, Mr. Epshteyn sent Mr. Giuliani an email recommending lawyers in those seven states.

As he had done in the earlier memo, Mr. Chesebro cited writings by a Harvard Law School professor, Laurence H. Tribe, to bolster his argument that the deadlines and procedures in the Electoral Count Act are unconstitutional and that state electoral votes need not be finalized until Congress’s certification on Jan. 6. Mr. Chesebro had worked as Mr. Tribe’s research assistant as a law student and later helped him in his representation of Vice President Al Gore during the 2000 election.

Calling his former mentor “a key Biden supporter and fervent Trump critic,” Mr. Chesebro cited what he described as Mr. Tribe’s legal views, along with writings by several other liberals as potential fodder for a messaging strategy. It would be “the height of hypocrisy for Democrats to resist Jan. 6 as the real deadline, or to suggest that Trump and Pence would be doing anything particularly controversial,” he wrote.

But in an essay published on Tuesday on the legal website Just Security, Mr. Tribe said Mr. Chesebro’s Nov. 18 memo “relied on a gross misrepresentation of my scholarship.”

For one, Mr. Chesebro quoted a clause from a law review article by Mr. Tribe about Bush v. Gore as support for the idea that the only real legal deadline is Jan. 6. That was taken out of context, Mr. Tribe wrote, saying he was only narrowly “discussing the specifics of Florida state law.” Mr. Chesebro, by contrast, made it sound as if he was putting forward “a general proposition about the power of states to do what they wish regardless of the Electoral Count Act and independent of the deadlines set by Congress,” he added.

For another, Mr. Chesebro cited a constitutional treatise in which Mr. Tribe wrote that a past Congress cannot bind the actions of a later Congress, which Mr. Chesebro used to buttress his proposal that parts of the Electoral Count Act are unconstitutional. But Mr. Tribe wrote that what he meant was Congress can pass new legislation changing such a law.

The indictment also accuses Mr. Trump and his unindicted co-conspirators of acting with deception in recruiting some of the fraudulent electors. That included telling some of them that their votes for Mr. Trump would be used only if a court ruling handed victory in their state to Mr. Trump.

The Dec. 6 memo dovetails with that approach. Mr. Chesebro wrote that Mr. Pence could count purported Trump electors from a state as long as there was a lawsuit pending challenging Mr. Biden’s declared victory there. But he also proposed telling the public that the Trump electors were meeting on Dec. 14 merely as a precaution in case “the courts (or state legislatures) were to later conclude that Trump actually won the state.”

Mr. Chesebro also suggested he knew that even that part of the strategy would draw blowback.

“There is no requirement that they meet in public. It might be preferable for them to meet in private, to thwart the ability of protesters to disrupt the event,” he wrote, adding: “Even if held in private, perhaps print and even TV journalists would be invited to attend to cover the event.”