Which party would theoretically benefit more from 100% turnout / mandatory voting?

[Former President tack50]

Basically asking this question as, for a long time, it was assumed automatically by most people that high turnout almost always benefited Democrats and low turnout almost always benefited Republicans; usually using the presidential and midterm elections between 2006 and 2018 as evidence.

However, one of the stories of 2020 is Trump easily beating his polls thanks to turning out a lot of people who normally do not vote.

The US election of 2020 still only had 66.7% turnout, meaning 1 in every 3 Americans did not vote. That is a lot of untapped votes; so many in fact that they'd beat Donald Trump for 2nd place nationally in the popular vote.

So, who would benefit if, for whatever reason, mandatory voting (like Australia or Belgium do) was implemented in the US and turnout shot up to 95% or something like that? Or even if, for some reason, turnout was magically 100%?

Would this benefit Republicans or Democrats? Would this effect be uniform or be concentrated in certain regions?

[Annatar]

Democrats would probably do around 1-2% better nationally with 100% turnout, higher turnout in 2020 did not help Trump, lower turnout would have been better for him. Higher turnout meant more 3rd party voters from 2016 that tend to be lower propensity voters turned out and most of these went for Biden, more young voters also turned out as turnout was higher.

The AP exit poll has Biden winning new voters 56-42, they were 15% of all voters, that is a 2.1% margin swing right there, without the huge influx of new voters, Trump would have won re-election.

[Hope For A New Era]

If it was mandatory? The incumbent party, because clueless people who don't care vote for the name they recognize.

[Red Wall]

The Democrats because they hold more low propensity voters. However even under mandatory voting there can be (and in lots of countries there is) rampant abstention. Low propensity voters are disinterested in politics, lots of them have less incentives and more barriers to get out and vote. Plus some times the fine is so ridiculously low people just pay it anyway. Even the poorest, which are the ones with the highest abstention rate.

[Skill and Chance]

Lean Republican in most of the country, though it would locally help Democrats in the South.

[Statilius the Epicurean]

It would likely benefit candidates who vaguely position as moderate on cultural issues and welfarish on economic ones. It would also probably benefit incumbents. Neither party would consistently benefit because low-propensity voters have no strong attachment to party ID.

[Coolface Sock #42069]

These days it might actually be Republicans since Democrats are now the party of college graduates, who tend to be more reliable voters.

[The Mikado]

Plus some times the fine is so ridiculously low people just pay it anyway. Even the poorest, which are the ones with the highest abstention rate.

I mean, yes, but Australia's 92% voting rate might as well be 100%. You get it that high and the difference between it and everyone voting is neglible.

[IceAgeComing]

and a large chunk of that 8% will either be people that planned to vote on the day but had a legitimate reason not to (things like a sudden medical emergency preventing you from planning an alternative way to vote) or people still listed on the electoral roll that can't vote: people who've died in the time between the roll being last updated and the election, or people that have moved away but are still registered at their old address.  A true 100% turnout isn't really possible: an official 92% turnout is in reality probably a percent or two higher than that.

[The Houstonian]

The Birthday Party.

I'm not being facetious. If everyone had to vote, joke parties would benefit the most.

[TML]

It mostly depends on where the additional turnout is coming from. If it's mostly from urban/suburban areas it will benefit Democrats, whereas if it's mostly from rural areas it will benefit Republicans. For example, in Pennsylvania, turnout increased over both of the last two election cycles; in 2016 it benefitted Republicans because most of it was in rural areas, whereas in 2020 it benefitted Democrats because most of it was in urban/suburban areas.