The original succession law, passed in 1792, designated only two people after the vice president: the president pro tempore of the Senate and the speaker of the House. In case either took over, a special election would be held to choose a new president. Why those two officials, and not someone from the executive branch — say, a member of the president’s cabinet? Because of raw politics: The secretary of state at the time was Thomas Jefferson, a prominent critic of the Washington administration, and the Federalists in charge of Congress weren’t about to hand him a potential pass to the White House.

This design had a fatal flaw, one that Americans today are watching play out in real time. In short, it’s a whopping conflict of interest for a lawmaker to be leading an impeachment inquiry that could result in her own ascension to the presidency. Consider what happened in 1868, when a Republican-led House of Representatives impeached President Andrew Johnson. In the Senate trial, one of Johnson’s most outspoken critics was Ben Wade, a Republican who also happened to be the president pro tempore. Wade voted to convict, along with 34 of his colleagues, one vote shy of the two-thirds majority necessary to remove Johnson from office. The vice presidency was vacant at the time, which meant Wade was effectively voting to make himself the president.

Whether or not Ms. Pelosi is actively thinking about her chances of ending up in the White House, the mere appearance of the conflict is bad on its own. That’s why Congress rightly changed the succession law in 1886, removing the speaker and president pro tempore from the line and replacing them with seven cabinet members, starting with the secretary of state. This arrangement lasted for 60 years, until President Harry Truman pushed to restore the speaker and president pro tempore — in the reverse order from 1792 — on the grounds that they are elected directly by the people, unlike members of the cabinet. The change was made, but this time with no special-election provision. With a few minor updates, this is still the law today.

So we’re back where we started — a line of succession that allows for the possibility that a member of the party that lost the last presidential election can assume the presidency.

This is terrible policy. The whole point of having a line of succession is to ensure a smooth transition and a continuity of administration in a time of crisis. Having a leader of the opposing party take over the White House, especially in an era of intense political polarization, would not achieve that, to put it mildly.

The succession law is also probably unconstitutional. Under Article II of the Constitution, only “officers” are eligible to serve as president. The framers almost surely intended to exclude legislators from that definition, as two constitutional scholars, Akhil Amar and Vikram Amar, pointed out nearly a quarter-century ago. Among many other pieces of evidence, there is a 1792 letter from James Madison, in which he criticizes the first succession law on the grounds that legislators are not “officers, in the constitutional sense” and that Congress “certainly err[ed]” by including them.

There are other big practical problems with the law. For one, even if lawmakers are not constitutionally ineligible to be president, they are without question barred from holding both jobs at once. So any lawmaker who may be called on to take over the presidency, even temporarily, would have to resign his or her office. It’s hard to imagine the speaker quitting for a two-week gig in the Oval Office. (And if the speaker declined, the job would go to the next person in line, although under the current law, the speaker or president pro tempore is allowed to “bump” that person out and claim the presidency later — another bad idea.)