That would be based on registered voters, and as anything to do with government a dubious stat.
There’s a song out by a young lady that has lyrics “...just because I said it don’t mean I meant it....”
It’s my statistic, actually, and it’s based on eligible voters, not on registered voters.
After the 2012 election, which Romney lost by 3.9%, I decided to calculate what the popular vote would have been if every eligible voter in the USA had voted.
I used the racial, gender, and age statistics collected by NEP, which is the exit pollster used by all major USA news media.
I used the eligibility stats calculated by Pew Research.
And I used the 2012 population stats from the US Census Bureau.
My result?
Romney lost by 4.1%.
In my personal opinion, that number is low, and is probably 5% or more.
Why?
Historically, white Conservatives turn out at a higher rate than any other ethnic or political group.
To me, that indicates that a 100% turn out would bring more Left wing voters to the polls, and fewer Conservatives.