Which president had the most consequential lame-duck period other than Buchanan?

[President Johnson]

Which president would you say had the most consequential lame-duck period in office? Ergo, the timespan between their successor's election and inauguration? The obvious answer is Buchanan in 1860/61, when Southern states began to secede. But since that's too obvious, I left him out.

There's a case for both Grant and Clinton to make, as the presidential election remained uncalled for several weeks after the election, even though the outgoing president didn't play an active role in the process. Poppy Bush is another contender, he made some diplomatic trips and signed treaties in early 1993, when he was already voted out. He also ordered, with Clinton's consent, a military operation in Somalia in December 1992. There's also a strong case for Trump 2020, obviously, as he refused to concede and ultimately was responsible for January 6.

[Orser67]

Adams is arguably more consequential than Buchanan. Not only did he appoint arguably the most important Supreme Court justice in history, he also wrapped up the Quasi-War (which was important in its own right, but also helped enable the Louisiana Purchase). He also presided over the first peaceful transition of power between presidents of a different political party, which perhaps feels a little less inevitable after 2020.

Hoover's lame duck period was also quite consequential as both the economy and the international situation (including Hitler coming into power) got worse, but the question seems to be about the presidents themselves rather than the time periods so he's probably not the best answer.

[buritobr]

Jimmy Carter 11/04/1980-01/20/1981?

Hostages in Iran

[○∙◄☻¥tπ[╪AV┼cVê└]

The lame duck amendment was ratified during Hoover's lame-duck period, but didn't shorten it.
There was election craziness in 1876, 2000, and 2020.