Yes, the Democratic Party could have been brought to the collapse point--by the actions of Republicans in the South during Reconstruction. If the Radical Republicans had treated the South as conquered territory, they could have combined white Southern states to reduce their congressional representation, split off unsettled areas such as most of Texas and southern Florida into either new territories to be settled by African Americans and immigrants or combined with northern and western states and territories. Western Texas and New Mexico, for example, could have been combined with Colorado by 1876 and South Texas split off into a new territory, while East Texas gets combined with Louisiana. Unionist North Alabama could be split from more confederate South Alabama, which could be combined with Georgia and West and north Florida. The Carolinas can be merged and even fused with Virginia (Virginialina?). While the heavily African-American Mississippi Valley can be pulled together in one, overwhelmingly African-American and thus safe Republican state comprising West Mississippi, West Tennessee, East Arkansas and East Louisiana, leaving West Arkansas and east Mississippi to be combined with West Louisiana into Texas and Alabama respectively.
And House Districts can be drawn on the basis of votes actually cast in the last general election in the decade, rather than the census of all PEOPLE, thus making any state which limits it's franchise in doing so, severely limit it's congressional representation, at least unless it either grossly stuffs it's ballot boxes or opens up the franchise with all the risk that entails every eight to twelve years.
This won't make the Southern Democrats disappear completely, buit will limit them severely, at least on the Congressional and Senatorial and Electoral Vote level.
If the Republicans, during the Gilded Age think that this will guarantee them permanent control of the nation, they are wrong, though. Because if the Democratic Party collapses as an effective political force, the Democrats will be replaced in the 1880s by the People's Party---the Populists---and possibly in the 20th Century by the Socialist Party. The US may wind up with Socialist Party urban machines, just like those of the German Social Democrats by the 1920s. While the rump Democrats have no choice but to become Southern Republicans--80 years earlier than IOTL. and who are the Tory alternative to Socialist Labor.
In short, a more honest political system with less pretense in which the parties mean what they say and say more of what they mean. Southerners who really stand for oligarchy will be calling themselves what they are--Republicans---just as they do now; since a republic is another name for government by oligarchy, be it a capitalist Republic or a Communist Party ruled nomenklatura oligarchy in a People's Republic. While Social Democrats or Socialists or Labor parties or Farmer Labor parties ala Minnesota and Wisconsin make no bones about what they want, both in terms of policy and voter participation. In short, a more honest if more polarized political system much earlier.
