"If Trump and Pence both get very sick, it’s not clear who would be president/President Pelosi? Invoking the Succession Act would lead to chaos."

That's the headline on a May 20, 2020 WaPo article by lawprof Sanford V. Levinson, which is getting tweeted about this morning as there's a lot of wild talk about Nancy Pelosi becoming the first woman President laterally — that is, breaking not the glass ceiling by opening the glass window that never had a working lock anyway.

It's good to have some sober, thoughtful, professional analysis standing by:

Should only the president become ill, then the vice president can take over, following the protocol laid out in the 25th Amendment. But if the vice president becomes incapacitated as well, then we could face a constitutional crisis. It would be triggered by the inadequacies of the Presidential Succession Act passed in 1947 (when there was no vice president, because Harry Truman had succeeded Franklin D. Roosevelt)....
Under its rules, the speaker of the House and the president pro tempore of the Senate would be next in the line of succession, followed by the members of the Cabinet, beginning with the secretary of state. Until 1947, succession had passed through the Cabinet. Congress added the speaker and president pro tem on the grounds that the president should desirably be an elected official, even if not part of the executive branch. This might make sense in theory, but it could be truly terrible in practice. Should both Pence and Trump be unable to serve, Speaker Nancy Pelosi (D.-Calif.) would become president under the act — handing the White House to a different party without an election. Should she be unable or unwilling to serve, then the office would go to Sen. Charles E. Grassley (R-Iowa).

Any effort to transfer power from Trump and Pence to Pelosi would surely inspire legal and political challenges, adding to chaos at precisely the moment the nation desperately needed stability....

Why? The act clearly designates Pelosi as next in line. Is Levinson saying that the Succession Act could be challenged as unconstitutional? He notes that Article II of the Constitution gives Congress the power “provide for the Case of Removal, Death, Resignation or Inability, both of the President and Vice President, declaring what Officer shall then act as President.”

It is not difficult to imagine that Trump would deny — and denounce as “fake news” — any suggestion that he lacks the ability, in the words of Article I, Section 2, of the Constitution, “to discharge the Powers and Duties” of the presidency.

That is, can the Succession Act apply? Congress has the power to provide for succession, but what is the scope of that power? You need an interpretation of the word "inability." The 25th Amendment is relevant to the question whether the President is unable, but what if both the President and the Vice President become deathly ill. Who says that there's inability when the effect would be to trigger the succession act and bring in the Speaker of the House — from the opposing party?!

The [Succession A]ct, first of all, bespeaks a simplistic theory of democratic legitimacy that ignores the prominent role that political parties — which have grown far more polarized since 1947 — play in the American system.

Pelosi might not even want the job, because it would require her to resign from her position in Congress, only to serve out a presidential term that ends in January. And what about Grassley? He's 86, and maybe he knows that, while he's okay hanging on as a Senator, he doesn't belong in the presidency.

There is also a serious argument, first laid out by Yale Law School professor Akhil Reed Amar and his brother, Vikram Amar, now dean of the University of Illinois College of Law, in a 1995 essay in the Stanford Law Review, that the Succession Act is unconstitutional. Article II specifically says that Congress — in setting rules of succession — must select an “officer” as a replacement for the president and vice president. Members of Congress, the argument goes, are not “officers,” because they are elected officials and not presidential appointees. (Another legal argument holds that the incompatibility clause does not apply if a member of Congress were to serve as president or vice president, because “officers” refers to people appointed by the president, not to the chief executive position itself. Under that interpretation, Pelosi could retain her legislative office, if the act were upheld as constitutional.) 

To put it mildly, it is hard to imagine these questions being litigated in real time should Republicans try to prevent Pelosi from taking office, or should she try to serve as president and speaker simultaneously.....

How would this play out? I assume there would be a tremendous resistance to getting to the point of saying both the President and the Vice President suffer from "inability." But here's an idea. The President could resign before things go too far, and that would make Pence President and empower him to pick a Vice President. That would go haywire however, because the appointment of a new Vice President, under the 25th Amendment requires "a majority vote of both Houses of Congress." Or do you think Democrats, for the good of the country, would confirm Pence's choice, if Pence too is ailing and approaching "inability"?

Whatever happens this time around, we should see the problem with the Succession Act. I think Levinson is right to propose, as he does, that we return to the line of succession that puts the Secretary of State after the Vice President.