The map below is one possible outcome. I’ll use it to discuss a range of different election outcome scenarios. Right now, Biden has just this a.m. pulled ahead in Wisconsin and Michigan, and he maintains his lead in Nevada. He also holds 3 of 4 votes from Maine (which splits its electoral college votes) and 1 of 5 votes from Nebraska. If Trump takes Georgia, North Carolina, and Pennsylvania, then the outcome is Biden 270 and Trump 268.
For a different outcome, Trump needs just one more vote. That would give him a tie 269 to 269, and the election would go to the House. There, each State gets only one vote, and there are more Republican controlled States, so Trump would very likely win.
If the election goes to SCOTUS (the Supreme Court), Trump has a 6-3 advantage of conservatives to liberals, but Justices are unlikely to have enough political bias to simply hand him the election. He could easily lose before SCOTUS.
A legal challenge of the Split-Vote system used by Maine and Nebraska would have no benefit to either side, as Trump would gain one vote and lose another; same for Biden.
It’s not clear what the basis would be for a legal challenge otherwise. Trump could refuse to concede until the very last ballots are counted. He could require a recount. I’ll have to research the recount laws in the States that are at issue.
Other Scenarios
If Trump loses PA, he needs both MI and WI to make up for it. If Trump loses NC, he needs MI for a 269-269 tie. If he loses GA, Trump needs both MI and WI to make up for it. If Trump can hold PA, NC, and GA, where he is currently ahead, he needs one more vote: MI or WI or NV. Arizona was called for Biden, but with only 84% of the vote in, that State could flip to Trump. Biden is only 18,100 or so votes ahead of Trump in Michigan, so Trump could still win there.
The main scenario that we should worry about is if Trump disputes a 268 to 270 loss to Biden, and refuses to step down. He could dispute the election based on the early mail-in ballots, which were heavily democratic in some States. The Constitution requires the Presidential election be held on the same day everywhere. But that would throw away so many votes, it might require a new election; it would not necessarily favor Trump.
If the election remains uncertain, the nation could erupt in violence. If either side wins in any way other than vote counting, same outcome. Violence offers no benefits to anyone in this election; it only harms the nation.
Recount Laws
NCSL.org says:
Michigan: candidates for office may request a recount; otherwise a 2000 or less vote margin triggers automatic recount.
Wisconsin: It’s not clear if the candidate can request a recount; probably a candidate would have to go to court to obtain an order for a recount.
Arizona: 0.1% vote margin triggers automatic recount; otherwise, you need a court order.
Pennsylvania: 0.5% vote margin for automatic recount; or, a voter petition; or “any three qualified electors” may request a recount.
Georgia: 0.5% vote margin with candidate request; otherwise, candidate request when paper ballots are used.
North Carolina: 0.5% of the votes cast in the ballot item, or 10,000 votes, whichever is less; otherwise, no recount.
Nevada: candidate or voters may request a recount.
Control of Senate
It looks like the Republicans might retain control of Senate.
Update
Wisconsin and Michigan have been called for Biden. Arizona should not have been called as there are still half a million votes uncounted, so Trump could still win Arizona, and therefore still win the election.
I’m concerned that the media will call Biden the winner before enough votes are counted to determine that. And then when the real count shows that Trump won, people will claim he stole the election.
RLCJ
the reproach of Christ 
Do you have any speculation on what group(s) would initiate a violent conflict, in response to a Trump loss of 268 to 270?
I don’t know. Maybe Antifa, maybe just people in general, Biden voters, protesting if Trump wins by SCOTUS or by a faithless elector.
It is sure to say that the decision will go to SCOTUS and very likely to the new Congress, which majority is still not clear.
Whoever wins, what we see is definitely not the way an election should be performed in 21st century in the leading democracy in the world. (USA is both republic and democracy. Notice there are monarchies in Europe that are democracies at the same time).
I wonder, how in the computer age and machines who read names and signatures one needs days to count as many votes as 6 million in one of the big states Pennsylvania? In comparison, any European election takes maximum 1 day to declare the winner, with exit polls giving preliminary but usually very correct results (1% margin of error) minutes after the election day is over.
If USA wants to be the leader in 21st century it certainly needs to change its current overheated mode, both domestically and internationally. Regardless of who is elected this time or the next time. What we see is not the image of a world leader who can hold high moral standards for other nations to follow.
The Catholics should help in that process and not making it impossible to be both a good Catholic and a good citizen.
If president Trump loses after all, I bet all those highly devote people who support him now will find solace in conspiracy theories and expectations of the end of time. Because frankly the Vatican did not do everything possible to disperse those theories. I said more about Fatima in previous comments. It is not enough to have a Vatican telescope in Arizona to know that the Vatican supports the science. What is the Vatican proposal for the near future, besides the supra national authority under the UN that will tax multinationals. The Vatican can’t afford to walk AFTER the events. None of that is a dogma, and I have the right to search other explanations. As the things are, without supporting the ultra conservatives, I myself feel dragged into the expectation of the end (of time, of political theater, of science that with all its telescopes cannot see an entire planet predicted millenia ago by Sumer texts). It is hard in such a chaos to see any other perspective than a certain preparation for the big end. Not end of the world but end of the era.