(a)An employee may not engage in political activity—
. . .
(2)
in any room or building occupied in the discharge of official duties by an individual employed or holding office in the Government of the United States or any agency or instrumentality thereof;
No, the President is not included under “employee” - but everyone else in the White House is.
Setting up a political rally on the White House lawn is most certainly a “political activity” - and every single person involved in organizing and setting up the event is “doing it”.
The debate was scheduled for Thursday, he should get a test Tuesday and another Wednesday. If they are both negative. He should call a press conference, walk out with a hammer and tack, tack them to the podium and walk off without saying a word.