For all of you saying that 24 years is too long (8 of which, mind you, were from elections where many Democrats were ineligible to vote), why isn't there such a problem with 28 to 40 years of one winning the presidency starting in 1800? Depending on whether you consider the Democrats and the Democratic-Republicans the same party, one of those numbers is valid.
Anyway, I find this scenario perfectly plausible. By whatever method (more radical reconstruction is the most obvious), the Democrats are utterly discredited as a party due to the taint if secession. A new party with a similar enough platform is formed, lets call it the Constitutional Federalist party (in opposition to the centralizing policies of the GOP, while also paying homage to the idea of the indivisible federal union). Dispirited Democrats join and they win the Preisdency, the final nail in the Democrats' coffin.
Challenge completed.