I would look more for voting place shenanigans, voter ID laws with only a weird subset of IDs allowed, radical gerrymandering, and stuff like that. Some of it will be blatantly partisan but also people are using justifications like "restoring trust in elections" to advocate for things that reduce the general franchise. They don't need to do a lot since a few percent is enough to swing the general balance of things.
What there are also going to be are allegations, claims of cheating, lawsuits...
Republicans currently control ~55% of all state legislative seats nationally and hold governing majorities in most states. In some of these states, they are incredibly partisan and just don't care - they will burn it all down to stay in power (MO, TX, etc.) Congress is required to certify the results of presidential elections, but state and congressional elections are a different matter. Those are certified at the state level, by state officials.
So what happens if Republican led states simply decide to declare, "We certify that the incumbent representative has been reelected," regardless of the actual vote count, or play other games e.g. discarding votes, eligibility, etc? It would be wildly illegal, of course. It would almost certainly trigger lawsuits, protests, and significant political repercussions within each state.
But here's the problem: in many of these jurisdictions, the federal district courts are controlled by Republican-appointed judges. The circuit courts are too. If the state officials certifying the results are Republican, the state courts and legislatures are friendly, and the federal courts that would hear any challenge are also sympathetic, how exactly would anyone stop them? Who enforces the ruling if the courts themselves are part of the alignment? The U.S. DOJ will not take up these cases on anyone's behalf. And in such a circumstance, you're unlikely to find a Congress willing to impeach the officials failing to do that.
More importantly, the National Guard and/or the Army gets activated under the Insurrection Act. So. That's the ballgame.
People keep saying "if" the election happens and it's definitely happening, it's just whether it's free and fair (the US has never had incredibly fair elections even in modern times, highly gerrymandered two party elections are really stupid, but substantially less fair than even the somewhat low bar the US traditionally sets for our democracy).
They'll be the most free and fair elections we've had since 2016, and maybe ever!
Let’s hope they are (happening).