As to the extra voting in Texas. 2020 had an additional + 1205300 Republican votes and + 1381258 Democratic votes in a solid red state which just hit 1/2 of voting age population. That’s 46.6% Republican and 53.4% Democrat and means those extra votes very much favored Democrats. Project a 20% increase in turnout with that kind of margin and Texas is in dead even recount territory.
Of course any election where Texas is that close Democrats have already won by such a margin as to make possibility flipping Texas largely irrelevant. Further, looking at these numbers it’s clear why targeted voter suppression becomes so significant.
You can’t conclude that because it overlooks third parties (who performed very well in 2016) and people who changed their votes from 2016 to 2020. That happened in suburbs all over the country. My traditionally red county (where voting was always super easy) went for Clinton by less than a point, but for Biden by 14 points. Voter turnout was up only 4 points, so Biden’s gain came mainly from flipping votes.
In Texas, Trump won basically the same share of the much larger turnout compared to 2016 (dropping just 0.1 points). Biden’s gain came almost entirely at the expense of third party candidates.
Indeed, nationwide, turnout went up 11 points. But Trump’s vote share also went up, from 46.1% to 46.8%. Again, Biden’s increased margin came entirely at the expense of third parties.
Donations to the Green Party for example are a clever and accepted means of reducing Democrats’s turnout. Libertarians draw surprisingly evenly from both sides, but Republicans have had their own spoilers.
Biden increasing his vote share at the expense of third parties doesn’t indicate any structural benefit to Democrats from higher turnout. It seems quite specific to Trump/Biden as candidates versus Trump/Clinton. Clinton and Trump had very high disapproval ratings, which pushed people to a surprisingly strong third party vote. Biden had much lower disapproval ratings than Clinton, which led those third party voters to vote for him in 2020. That doesn’t show a structure benefit to Democrats from higher turnout.
I’ve done a little numeric modeling, and it produces some surprising implications about who new 2020 voters selected. Pre-election polling showed that 9% of Trump 2016 voters planned to vote for Biden, while only half as many Clinton 2016 voters planned to vote for Trump. Moreover, they showed people who voted third party in 2016 strongly favoring Biden.
Trump won about 11 million more votes than in 2016, and Biden won about 15 million more votes than Clinton. But there was an over 3 million vote drop in the third party vote. If you assume that Biden pulled in a significant number of Trump 2016 votes, say 6-8%, while losing only 3-4% of Clinton 2016 voters, and won the majority of people who voted third party in 2016 but not in 2020, then its quite likely Trump actually won a majority of the 22 million new votes in 2020.
Your nationwide numbers are also off for several reasons. For example many people voting in 2016 skipped the 2020 elections for various reasons like death. Further pooling including exit polls doesn’t match up with actual voting very well suggesting a significant portion of Trump voters are unwilling to say they voted or plan to vote for Trump. That or a truly massive pro Republican voter fraud which seems unlikely.
Anyway looking at hard data. In 2016 Clinton got 65,853,514 votes Trump got 62,984,828 votes. A Democratic margin of 2,868,686 million votes. In 2020 Biden got 81,282,896 votes vs Trumps 74,222,484 a Democratic margin of 7,060,412.
Using the middle of your range 62,984,828 * 0.075 - 65,853,514 * 0.035 that’s a net gain of 2,418,989 votes which far less than the actual net gain of (7,060,412 - 2,868,686) = 4,191,726 votes that Biden over Trump.
To gain the (4,191,726 - 2,418,989) = 1,772,737 votes from a 3 million 3rd party voter drop would have taken a truly extreme margin of ~80% even assuming your other assumptions where correct. That might be reasonable if they where mostly green voters, but it’s largely from the Libertarian party.
And that’s exactly what you would expect from the data. Georgia is one of the few states in the country where Black voter registration rates are equal to registration rates for whites: https://www.kff.org/other/state-indicator/voting-and-voter-r.... (California, by contrast, has a 12 point difference in Black-white registration rates.) In 2018, the Black voter turnout rate was actually 2 points higher than the white voter turnout rate in Georgia. It makes total sense that as you increase turnout, you’re going to see roughly the same demographics.
Same thing in Texas. 87% of votes were cast by mail in ballot. Turn out was up almost 7 points compared to 2016. But Trump earned basically the same percentage of the vote as in 2016, with Biden gaining a few points from winning people who voted third party last time: https://www.texastribune.org/2020/11/04/texas-voter-turnout-...
Democrats have this idea that voter suppression is preventing more minorities from coming out (and voting Democrat). But what the data seems to suggest in Georgia and Texas is that people who want to vote are already voting. Making it easier to vote or having a high states election will increase turnout, but by dipping into a pool of lower propensity voters who don’t look all that different demographically than the people who were already voting.