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If as you claim that it is usually Republicans, then why is the Republican party advocating anti-fraud measures, while the Democratic party is downplaying its very existence?
You've presented evidence that voter fraud exists at very, very low levels. Handfuls. Not enough to affect a nationwide election.
You've claimed it could be "millions", but presented no evidence. Trump claimed the same thing, has zero convictions to show for it.
There are certainly isolated examples that can affect a local election. Not the topic of this thread. Impossible to sustain for long without being caught. Penalties are steep, not worth the risk unless you own the local law enforcement officials.
Why are Republicans usually the ones getting caught?
Who was the famous wit who said not all Republicans are stupid, but all stupid people are Republicans? Maybe a little harsh, but there's a grain of truth there. Even more so, the entitled elite who have gotten away with anything and everything their whole lives are more likely to be Republicans. They don't think the rules apply to them. Occasionally they find out that voting in two different states is illegal.
Why do Republicans whine incessantly about "widespread" voter fraud while simultaneously being unable to find, charge, and convict these "millions" of fraudulent voters even while controlling the entire criminal justice system?
The answer is that Republicans are
not actually worried about widespread voter fraud. They are using it as cover to disenfranchise voters who are likely to vote Democratic. Simple as that. Every damn time. See Florida. See Georgia, Texas, MS, AL, WI ... etc.
When Republicans control government, they make it harder to vote. These hardships disproportionately fall on Democratic voters, by design. This benefits the Republicans in power.
When Democrats control government, they make it easier to vote. This benefits the party that can get the most votes. Often that is Democrats.