My guess is 30 to 40 % Reps leave, 5 to 15 % Dems leave, and a whole bunch of 'independents' join as well, various libertarians and crypto/techbro type people.
EDIT:
Yay, I don't have to do any too much math, they actually did most of the math.
Apologies for shitty cell phone image, here's the whole poll (3rd link in the article, by the way):
https://drive.google.com/file/d/1R4pZVo0ZnrQyElZQzdNtt7CQ1zwTypuS/view
...
So, ballparking a 50/50 male female split:
Likely to Join Elon's Party:
Republicans
Very likely: 23 + 14 / 2 = 18.5%
Somewhat likely: 34 + 29 / 2 = 31.5%
Not likely: 31 + 30 / 2 = 30.5%
Independents
Very likely: 18 + 11 / 2 = 14.5%
Somewhat likely: 29 + 26 / 2 = 27.5%
Not likely: 35 + 36 / 2 = 15.5%
Democrats
Very likely: 7 + 5 / 2 = 6%
Somewhat likely: 15 + 16 / 2 = 15.5%
Not likely: 54 + 48 / 2 = 51%
...
So, if you say half of the 'somewhat likely's actually go for it, then you get this:
Republicans who join Elon Party: 34.25%
Independents who join Elon Party: 28.25%
Democrats who join Elon Party: 13.75%
...
So yep, my ballpark guess was indeed in the correct ballpark.
...
Worth mentioning:
There are more Dem voters, than Republicans.
But there are also more independents than either.
Roughly 32% Reps, 33% Dems, 35% Indp.
https://www.pewresearch.org/politics/2024/04/09/the-partisanship-and-ideology-of-american-voters/
So, throw that in with those previous calcs, and you end up with:
Reps: ~21%
Dems: ~28.5%
Elons: ~25.5%
Indps: ~25%
or, normalized to remove remaining Indps:
Reps: 28%, Dems: 38%, Elons: 34%
So... theoretically, the Dems are still the largest, Elon is now second behind them, and the Reps are now a third party, less popular than having no solid political affiliation.
... if this actually happened, which it could, Elon has enough money to single handedly start a party, though he'd have to find some actually competent people to... do anything other than spend money...
I think you end up with a good number of corpo Dems leaving the Dems, so the Dems now have an easier time shifting to the left.
The Reps lose 1/3 of their voters, and basically just become a cult of idiot racist nazis, paleocons, theocrats, MAGA nutjobs.
Elon party ... basically becomes the 'centrist'/libertarian/ancap/corpo party.
... and everyone would now have to figure out how to do politics in a much more complex kind of paradigm.
I... don't think a roughly even balanced 3 party system has ever existed in US history with any kind of stability, that endured more than one or two Presidential elections, 8 years.
Two roughly approximate examples:
The Civil War.
Teddy Roosevelt going Bull Moose Party.
https://en.wikipedia.org/wiki/Political_eras_of_the_United_States
But uh yeah, not stable.
And these numbers are close enough that a 3 party system might not implode quickly... or it also could.
Especially if the President just gets to remain as functionally a dictator as the last months and years Supreme Court rulings have made him.
???