On: Joint Chiefs remind U.S. forces that they defend the constitution

I unfortunately am not able to find the source. Basically, there were provisions for resigning vs. impeachment. Nixon resigned before impeachment.

Trying to scour wikipedia over this. I'll comment here if I find the source.

For completeness another interesting case is Grover Cleveland. He served two terms, but they were not consecutive.

Some lists count him as both 22nd and 24th, and some count him as just 22nd.