A day before a mob of Donald Trump supporters smashed their way into the Capitol to disrupt the transfer of presidential power, then-Vice President Mike Pence's top lawyer dashed off a fateful memo.

In the three-page document, attorney Greg Jacob concluded that if Pence were to embrace Trump's demand that he single-handedly block or delay the counting of electoral votes on Jan. 6, he would be breaking multiple provisions of the Electoral Count Act, the law that has governed the transfer of power since 1887.

Such a move, Jacob concluded, would assuredly fail in court. Or worse, he said, the courts would refuse to get involved and leave America in an unprecedented political crisis.

In that case, he said in the memo obtained by POLITICO and published for the first time, "the Vice President would likely find himself in an isolated standoff against both houses of Congress ... with no neutral arbiter available to break the impasse."

Jacob is scheduled to testify publicly Thursday to the Jan. 6 select committee about Pence's decision to resist Trump's pressure campaign. The panel declined to comment on Jacob's memo.

The memo informed Pence's ultimate decision to rebuff pressure from Trump to reverse the outcome of the election. Pence announced his decision the next day, when he traveled to the Capitol to preside over the Jan. 6 meeting of the House and Senate. His decision, in a letter that closely tracked Jacob's memo, inflamed a crowd of thousands of Trump supporters that the president had called to Washington to protest his defeat.

Within an hour of Pence's announcement, hundreds of members of that mob would bludgeon their way past police lines and into the Capitol itself, sending the vice president and members of Congress fleeing for safety. Some members of that mob chanted, "Hang Mike Pence."