(https://en.wikipedia.org/wiki/2004_United_States_election_vo...) has a pretty good summary of the issues. The voting machine claims were a big deal at the time; I remember a lot of news coverage about that the voting machine companies were owned by Republicans.
I think the 2016 fraud claims were about Russian hacking (https://en.wikipedia.org/wiki/2016_United_States_presidentia...). (https://blogs.lse.ac.uk/usappblog/2018/09/11/many-democrats-...) claims that many Democrats questioned the 2016 election.
> On the left I see claims of disenfranchisement done through legal means, which whether you agree with or not is at least something real and concrete.
I agree that disenfranchisement is an issue and am in favor of federal voting standards and protecting the voting rights of convicted felons.