I am sure that Trump had in mind how honest the GOP was when, and if, he signed anything which is highly unlikely. The GOP has been utterly deceitful and any promises made to them have been completely sundered by GOP faithlessness and treachery.
Even if such a paper miraculously exists it is legally unenforceable because nobody can sign away their right of free association and President dump can always claim that the new party drafted him.
Spell checker?? Anybody seen the spell checker???
No controlling legal authority... :)