If Trump wins, expect it to be much, much worse. The ACA/Obamacare guarantees that certain things must be covered, that you can't be denied covered based on pre-existing conditions, and that you can't be charged more due to age, gender, etc. It also gives subsidies to people that are buying their own covered on the marketplace, which was set up by the federal gov't.
Under Trump, expect all of that to be tossed out. If Trump wins, it's highly likely that Republicans with flip the Senate, and retain control of the House, which means Republicans will have all three branches of the federal government captured, and there will be no brakes to repealing the ACA and going back to the old, much shittier system.
If Harris wins, don't expect to see many changes. If she wins, it's unlikely that Democrats would also have control of both the House and Senate. While it's true that she was in favor of a single-payer system five years ago, it's unlikely that she would be able to get that through the House and Senate unless they were both controlled by a Democratic majority. (In the case of the Senate, they would need to nuke the filibuster, which--IMO--is not a good idea in the long run).