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>the House and Senate have republican majorities as I understand . Why isn't voter fraud an issue in those elections.?

While I'm not going to say that the Rs have been pristine in the area of voter/ballot fraud, over the last 8 years it's been more common on the D party side than the R party side.

Historically it is more severe in local elections than national ones.

This is actually a serious issue - though some are confusing instances of administrative errors with cases of actual fraud/misconduct.