. . . despite a record of accomplishment that could fairly be judged as mediocre.
The only Democrat president who came close to the mediocre standard in the last century was JFK. And yet that still makes them outstanding as Democrat presidents go.
FDR's long praised leadership skills for leading us during the war is more than cancelled by his inept prolonging of the Great Depression to the extent that it required a war for us to make a full recovery and his treaties for postwar giveaways of vast tracts of territory to a Communist enemy.
Nixon supposedly left office broke due to Watergate. It might be interesting to compare his financial records when leaving office with Truman. Nixon’s situation is somewhat believable since it was so fashionable then to hate him (and still is !). I do caveat this with the fact that Nixon was a professional politician and few if any of that species leave office in worse financial shape then when they enter.