Birtherism is of course just racism, saying these black candidates are different, and suspect Americans because of their skin color. Nobody really thought Obama or Kamala wasn't an American citizen. Well, some people are stupid and believe all sorts of nonsense. But it's all cynicism and dog whistling.
Birtherism is what got Trump political attention to begin with and was essentially the foundation of his political ascendancy. Of course he's going back to the well. Which also includes:
1) Getting Russian help.
2) Hacking into Dem servers to leak behind the scenes info.
3) Calling your opponent corrupt and that he/she should be jailed.
4) Mocking your opponent ("Sleepy Joe" hasn't really caught on or been damaging).
5) Questioning your opponents mental health (remember all that rigmarole about Hillary having hit her head and having cognitive difficulty?)
6) Racist appeals
7) The tried and true GOP tactic of suppressing the vote; with Trump adding the Post Office wrinkle.
an October surprise -- investigation, indictment, report about wrongdoing
9) signaling possible unwillingness to accept the results
I'm sure there are more.
So far all of the above are being deployed. The only one we haven't heard about yet is hacking your opponent's email. Which you can be sure is being attempted. And in this Distanced Campaign more info than ever is being communicated by email and messaging apps.
One new wrinkle is the possible presence of armed brownshirts at polling stations. Between armed Trump protesters, trying to use the military on the streets on America, and using Customs/Border agents and other enforcement agencies in US cities, there's a very dangerous and uncomfortable element of state sanctioned violence as background threat to this election.