Well, that is the point, it IS constitutional. It may be undemocratic, unfair or politically motivated (I will get to that later) but the original article was drafted by very well-respected constitutional professors and members of the Federalist Society.
Do not hold your breath hoping for SCOTUS to dismiss this. Whilst it has been Dems who have brought the court cases, it is the GOP who secretly wants them to succeed. Most Democrats I talk to want to beat him fair and square, despite the fact that the last time they did so, he ended up attempting a coup.
Let us be honest, if he dropped dead tomorrow of the 5th hole of Bedminster. The collective sigh of relief from 95% of the GOP (including Tucker) could power a wind farm. This gives them a get-out clause that does not require them to grow a spine. I predict that SCOTUS will let this stand but will grant him presidential immunity. Trump will be told to accept it as the best deal he can get to keep him out of jail. Which is let's face it, the only reason he is running.