@digital-wisdom is quite right to acknowledge that this election is far from over and the GOP has Constitutional options available to it that would re-elect Trump.
The Democrats should have thought about this before engaging in fraud and illegal behaviour and then having their mates in the MSM anoint Biden as "President-Elect" when the results are far from certain and we are 8 weeks from anyone having the right to use that term.
They should also stop being so vindictive in "victory". Threatening that anyone who has supported Trump won't ever work again and saying to major US allies that "you picked the wrong side" is not going to calm things down.
Every democracy understands that mail-in votes are problematic from a fraud perspective. For a small percentage of special circumstance votes they may be OK, but not when its a large percentage of the total vote. Israel doesn't allow them at all and Australia only allows them in quite limited circumstances.
The State Legislatures have not only the Constitutional right, but the obligation, to make their own decision on the true will of the people when the vote count is either undecided or irrevocably tainted by fraud or illegality.
There is nothing wrong with them doing this. They were elected by the people of their State (in untainted elections) and thus their decision IS the will of the people of their State.
The Constitution, not custom, governs what is right and wrong in this situation. The Supreme Court has been very clear in old and recent decisions that they have every right to do this. Read the 2020 Supreme Court decision Chiafalo (about faithless electors in 2016) for the law and history of this issue.
There is plenty of historical precedent for this and it almost happened in 2000.
There will be no civil war because empty vehicles and mail-in ballots don't fight wars - people do. Trump has demonstrated his ability to draw crowds of hundreds of thousands of people to his rallies all around the country while Biden struggled to get a couple of hundred.
If you read the history of US elections you'll see that the USA has survived through much more hairy situations than today by staying faithful to the Constitution.