Former Vice President Mike Pence’s chief counsel laid out in a memo the day before Jan. 6, 2021, that the vice president would violate federal law if he bowed to pressure from President Donald J. Trump to interfere with the certification of Joseph R. Biden Jr.’s victory.

The three-page memo, obtained by Politico and confirmed as authentic by The New York Times, included arguments from the chief counsel, Greg Jacob, that Mr. Pence could find himself in a legally precarious situation if he decided to block the certification of the Electoral College results either unilaterally or by calling for a 10-day delay in the proceedings.

A lawyer advising Mr. Trump, John Eastman, had insisted that Mr. Pence had the power to take both of those actions, emphasizing the 10-day delay as Jan. 6 grew closer. Mr. Eastman pressed his claims in a meeting with Mr. Pence and Mr. Jacob in the Oval Office on Jan. 4.

But Mr. Pence, who in the weeks after the election told Mr. Trump that he did not believe he had such power but would continue researching the matter, was given concrete guidance by his own aides.

Mr. Jacob wrote in the memo that Mr. Pence would most likely be overruled by the courts if he made such a move.

“In a best-case scenario in which the courts refused to get involved, the vice president would likely find himself in an isolated standoff against both houses of Congress, as well as most or all of the applicable state legislatures, with no neutral arbiter available to break the impasse,” Mr. Jacob wrote in the memo.

A spokesman for Mr. Pence declined to comment.

Following its prime-time hearing this past week, the House committee investigating the Capitol riot is scheduled to hold three more hearings in the coming week, including one on Thursday at which Mr. Jacob is set to be a key witness.

That session is slated to focus on the pressure campaign on Mr. Pence to insert himself into the certification of the Electoral College votes, a proceeding that is usually routine.

Mr. Jacob has told the committee that he wrote the memo after the meeting with Mr. Eastman, Politico reported.

Mr. Eastman’s conduct has been a focal point of the House investigation into the events that took place leading up to the riot. In March, in a civil case related to Mr. Eastman’s efforts to keep the committee from accessing a tranche of emails related to his advice to Mr. Trump, a federal judge said that he and Mr. Trump “more likely than not” committed crimes as they sought to overturn the results of the election.

The memo from Mr. Jacob was one in a series that he wrote related to the pressure on Mr. Pence following the 2020 election. One came at the beginning of December, after Mr. Pence asked Mr. Jacob to explore what his authority was in relation to the Jan. 6 certification.

Another memo, also obtained by Politico, was written on Jan. 1. It evaluated the various allegations of widespread fraud that Mr. Trump’s advisers had pointed to, including in Georgia, where Mr. Trump repeatedly made claims that officials said were baseless.

The memo detailed the claims from six key states — the ones for which Mr. Pence’s advisers anticipated that House lawmakers would try to challenge the certification, potentially with support from senators from those states.