The White House keeps a visitors log, and it is that visitors log which will probably be used to help impeach Biden. He claims he had no knowledge of his son's business dealings yet while he was Vice President under Obama, White House visitor logs show his son's business partners logging in to visit with Biden or with Biden's secretary.
At Biden's home in Delaware there are NO visitor's logs allowed, which is problematic because he spends most of his time there.
In the end, the only witnesses to his Delaware visitors will be the Secret Service, and I doubt they will be allowed to testify if any of the Delaware visitors are of a compromising nature.
