Did emergence of the modern party system cause the decline in Presidential quality after Jackson?

[Unconditional Surrender Truman]

If we're talking specifically about the antebellum presidents, I would argue a far more significant factor was the unwieldy nature of both major party coalitions (particularly the Whigs), necessitating nondescript, preferably apolitical nominees who could appeal to all elements of their party. Hence Clay was passed over for Harrison (who walked into the River Styx and reemerged as the nonideological Hero of Tippecanoe) in 1840 and Taylor (who had no opinions and had never voted in his life) in 1848, Webster denied the nomination in 1836 (for being too northern) and 1852 (for being too southern), Van Buren was dumped for Polk in 1844, and so on. The "great men" of the age simply had too many enemies to be elected, whereas the parade of war heroes and dark horses who governed from 1837-1861 were too anonymous to be reviled by anybody, at least until after the election. Maybe this cost us a few "great" presidents in among those twenty-odd years, but on the other hand the same impulse also gave us Abraham Lincoln, who was so obscure that newspapers announcing his nomination initially identified him as "Abram" Lincoln.

But I think another factor might be that the politicians of this era—presidents or otherwise—tended to be short-sighted in ways that are painfully obvious to us who know what was waiting at the end of the antebellum. Had Washington or Jefferson served in the second quarter of the nineteenth century, they would not be the beloved figures they are today: the antebellum was a stink that clung to everyone who lived through it, and even giants like Clay and Webster come off looking pretty bad in hindsight. It was an age when "narrow sectional and ideological prejudices" prevailed, and the shoddy presidents who characterized it were the symptom rather than the cause of its degeneracy.