turnout is always < 1.00 in the US because voting is not mandatory and registration is not (universally on a national level) automatic—which would need to happen before mandatory voting.
so it is theoretically trivial, once you know how many votes you needed to have won that you did not, to add a number of fake votes N to the number of real votes P such that N+P < T, the total of registered voters.
however, this is actually not that easy to do en masse scale because of how discretely elections are conducted. people in the US generally vote by precinct (determined at the local level by the county board of elections) which is about the size of a census tract, which is a relatively small area. it is basically impossible to know ahead of time which specific precincts you need to flip in order to change the result in a credible way. injecting the ballots into the process would require a level of sophistication that interacting with the county election officials or volunteer election administrators would lead you to believe is not possible.
the actual way to do election fraud would be to just report vote totals that have no basis in fact. which, sure, i guess is also possible. but then, while Democrats control the urban centers so may have more registered eligible voters who do not vote to be able to fake vote totals within their cities, Republicans control the vast majority of counties in the country. so at scale, effective use of this strategy would just result in everyone reporting 100% turnout every year...which clearly doesn’t happen.
the fact that any arms race here would rapidly devolve into a situation neither side can accurately predict means that election fraud at scale not only does not occur, but also that is is actually not preferable. it is way easier to play in other margins (make it harder for people to register, kick people off voter rolls, etc).
so it is theoretically trivial, once you know how many votes you needed to have won that you did not, to add a number of fake votes N to the number of real votes P such that N+P < T, the total of registered voters.
however, this is actually not that easy to do en masse scale because of how discretely elections are conducted. people in the US generally vote by precinct (determined at the local level by the county board of elections) which is about the size of a census tract, which is a relatively small area. it is basically impossible to know ahead of time which specific precincts you need to flip in order to change the result in a credible way. injecting the ballots into the process would require a level of sophistication that interacting with the county election officials or volunteer election administrators would lead you to believe is not possible.
the actual way to do election fraud would be to just report vote totals that have no basis in fact. which, sure, i guess is also possible. but then, while Democrats control the urban centers so may have more registered eligible voters who do not vote to be able to fake vote totals within their cities, Republicans control the vast majority of counties in the country. so at scale, effective use of this strategy would just result in everyone reporting 100% turnout every year...which clearly doesn’t happen.
the fact that any arms race here would rapidly devolve into a situation neither side can accurately predict means that election fraud at scale not only does not occur, but also that is is actually not preferable. it is way easier to play in other margins (make it harder for people to register, kick people off voter rolls, etc).
source: worked on campaigns in another life.