Matt Seligman, Senior Counsel at the Campaign Legal Center, has this 33-page legal analysis on this question. Here’s the abstract:

This Essay explains the errors in the theory advanced by John Eastman in a memorandum presented to President Donald Trump and Vice President Mike Pence which claimed that Pence had the unilateral authority to reject electoral votes when Congress convened on January 6, 2021. That Unilateral Rejection Power theory is fatally flawed in four respects. First, it rests on a misreading of the critical phrase in the Twelfth Amendment that does not follow from its text. If anything, the text indicates the Unilateral Rejection Power is incorrect. Second, it draws on an erroneous history of the Vice President’s role in the electoral counts of the 1796 and 1800 presidential elections. Third, it ignores the dispositive drafting history of the Twelfth Amendment, which definitively demonstrates that both chambers of Congress understood Article II, section 1, clause 3 to assign to Congress rather than the President of the Senate the power to count votes and decide disputes relating to that count. Congress drafted the Twelfth Amendment using precisely the same words regarding the process of counting electoral votes, thus incorporating that settled understanding. Finally, it defies reason that the Founding generation, who had just fought the Revolutionary War to overthrow a monarch to establish a representative democracy, would create a constitution that vested a single official with the legal authority to retain power for a lifetime.

This Essay offers these legal arguments in full view of the more fundamental point: that the Unilateral Rejection Power theory is a dangerous view that undercuts the basic principles of American democracy. It may seem that nothing more than that needs to be said—and that even engaging in rigorous legal argument with such a radically antidemocratic interpretation of the Constitution legitimizes a view that should simply be shunned. The tragic reality, however, is that radically antidemocratic legal views like the Unilateral Rejection Power theory have perilous influence with a powerful faction in the American political system and thus present an immense danger if not decisively rebutted. For that reason, I believe it is critical to expose that the Unilateral Rejection Power theory is not only gravely morally wrong—it is, beyond any doubt, legally wrong as well.