Considering making some wikiboxes for an alternate 1964 election where early polling showing Johnson beating Goldwater 77% to 18% is accurate. The polls by the end of the election were off by around 6%, so I'm figuring a Johnson win of around 71%, a swing of around 20% from OTL
Figured I'd start off making some congressional maps. Not sure if that full swing of 20% would be seen downballot, so first, here's a map where the Dems get a 10% swing from otl in the House. It isn't quite the apocalyptic map for the GOP that might have been...
...with just a loss of 85 seats, not even as bad as 1932 was for the GOP. Nonetheless it is a very big win, with the Dems ending up with 343 seats (48 more than OTL), and the GOP put below 100 seats, holding just 92 seats. They still flip some seats in some states in the ex-confederate states, the only area (apart from an instance of redistricting in Wisconsin) they flipped any seats, but even there, they lose seats in more states than they gain seats, and the number of seats they flip is equal to the number of seats they lose. Popular vote was 62% D to 37% R
In the Senate, the Dems hold California and flip every single seat held by a Republican except Nebraska, bringing their total up to 74 seats with a gain of 8...
Now, with a 15% swing, the Dems get the largest swing in US House history, narrowly beating out the 1894 GOP gain of 111 seats (though with it being ever so slightly less in terms of the percentage of House seats flipped, since there were fewer seats in 1894) by having a net gain of 114 (77 more than OTL) seats, for a total of 372 seats. The GOP hold only 15% of the House, with just 63 seats. Popular vote is about D: 64.5% to R: 34.5%

The Senate remains the same as the +10% map
And with a 20% swing (along the lines of the Presidential election in this scenario)...

...absolute carnage. A popular vote win of around 67% D to 32% R, for a net gain of 139 for the Dems, 102 more than OTL and by far the largest House gains both absolutely and in terms of percentage flipped. Dems end up with 397 seats, while the GOP end up with 38, under 10%
And the Senate still remains the same, though the Dems come within just a couple percent of flipping Nebraska and thus winning every single senate contest in that year
Any thoughts on what would happen in scenarios like these? Would the Great Society end up much more ambitious, perhaps with some sort of single payer health program, a national education program, and some other ideas like a universal basic income and a push for full employment (that goes further than the Humphrey-Hawkins 1978 act), or at least some of those things? The Democrats would hold very large majorities in even the most minimal situation, but on the other hand, the moderate to conservative southern Democrats remain a potential issue, and some of the seats gained in the north are just seats flipped from liberal Republicans anyway (though it could be possible that a Democrat in those areas could still be more willing to sign on to, say, single payer healthcare than even a Rockefeller Republican in some cases). At any rate, I'd imagine the Great Society would end up somewhat more ambitious, though it could still fail to live up to progressive hopes regarding things like healthcare
And what does the Republican Party do? They retain, even in the worst of scenarios, a core of around a third of the electorate, and could still be poised to make some huge gains in 1966, without Goldwater dragging them down, which could get them to about where they were in OTL, though the senate elections would make things harder for them there over a longer period. But could they survive such painful losses in the short term? I am having trouble finding references to it, but I recall reading somewhere that Reagan had floated the idea of renaming and rebranding the GOP after Watergate and the 1974 election losses, but that never ended up happening. Here, though, the losses are worse than in 1974. Could that perhaps cause some sort of split or chaos among the different wings of the GOP, or would it just end up with them bouncing back about as well as they did in OTL, perhaps just a bit worse due to more Dems with incumbent advantage, or perhaps it holds about even due to a more ambitious Great Society generating more midterm backlash?
Also, I wonder if these results plus a more ambitious Great Society could lead to a somewhat different 1968 election that leads to more of a southern split. In OTL, Wallace split off to form the American Independent Party, but even Congressional southern Dems who supported him didn't split off to join him, nor did any of the AIP candidates for Congress get more than single digit percentages in any districts, for a national total of just 0.3% of the popular vote. I wonder if Wallace could end up with more support, both personally and with more congressional Dems splitting off and performing better as third party candidates, with the GOP having suffered such a big defeat, with Wallace perhaps being able to be more credible among conservative voters than a Republican