Duverger's Law in the United States: Who's on the chopping block?

Assuming that Duverger's Law holds true in the United States, and, assuming that a third party in the United States moves up the political ladder sometime in the 20th Century or early 21st Century, which traditional American political organization bites the dust? Below are some of my personal theories as to what might end up happening.

Populists replace the Democrats. Assuming the Populist Party does better in an ATL, I think it takes place of the Democrats as the key opposition party in much of the United States. The Democrats of the late 19th and early 20th Century were quite weak compared to the machine that was the Republican Party, and as Populists will be able to co-opt western and Southern support from both parties, I think that ultimately they do so at the expense of the Democrats, because of the sheer numbers of urban workers and industrialists in the north that aren't going to be voting Populist anytime soon if OTL's performance(s) are any indicator.

Progressives replace Republicans. They've got essentially the same base, so I can see the Progressives eventually taking most of the Republican Party while the Democrats become the de facto conservative party in the North and in the South, uniting OTL's Republican base nearly seventy years early.

Socialists replace Democrats. Should a Socialist Party rise up in the United States, I think it does so at the expense of the Democrats. Northern workers will be more predisposed to voting Socialist than Democratic, along with the Democrats' traditional urban and immigrant base. Republicans aren't going to be winning the South anytime soon, but they'd probably be able to start carrying Southern states (borderstates, anyway) a few decades after the first Socialist electoral victory. As blacks gain the right to vote in the South, expect them to vote heavily for Socialist candidates.

'American Independents' replace Republicans in the South/Southwest. Assuming that both parties support Civil Rights and neither play up blacklash politics, I think that George Wallace's movement could gain more steam in the South and take what's left of conservative Democratic voters to keep control of Southern politics, leaving the southern Republican mordibund. Of course, I think Duverger's law would again eventually mean the death of this movement sometime around the early aughts in favor of a new two-party system in the South between younger Republicans and Democrats, as well.

The Reform Party replaces the Republican Party in the midwest. Assuming the Republicans go right as IOTL and that, unlike OTL, Ross Perot focuses on building a party that can compete at the state level, I think that the Reform Party can displace Republicans in the midwest, which is generally moderate Republican territory. Not to say that Republicans won't win elections here, either, but I think it's a safe bet that 'Reformers' would benefit from the Republican Party's rightward shift.
 
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