Dandy I worked under the Hatch Act for 35 years, and up until now it would never have happened. And that's because no previous POTUS would ever have abused the privilege of being exempt from the Hatch Act in this manner. Scrotus is trampling on 'invisible lines' that have been in place since 1939, when the HA was passed.
The exemption is based on POTUS/VP being able to do things that 'run of the mill' Govt employees can't like accept 'official' gifts, be wined and dined, or to 'publicly' endorse a partisan political candidate. Under the Hatch Act govt employees are not allowed to do any of these things (plus more). 'Regular' employees can say we like XXX candidate privately but are not allowed to speak at a rally, take part in publicity (ads), or publicly campaign in any way (ie door to door, leaflets etc). A POTUS is allowed to publicly endorse.
One of the great things about being retired is being allowed to actively campaign and work for partisan candidates.
Technically there is a law that bars federal property from being used for partisan politics. This is separate from Hatch. So the use of the WH is improper based on that alone, regardless of who is organizing or who is speaking.
The primary purpose of the Hatch Act is to 'Prevent Pernicious Political Activities' (it's actual title is 'An Act to Prevent Pernicious Political Activities'). It was originally meant to prevent acceptance/offering of bribes and to prevent 'higher ups' from forcing/using pressure on 'lower downs' to support or vote for a particular candidate.