Saying Trump is "of their own party" has always been a stretch. 

While it is *technically* correct that Trump is a registered Republican and ran as the (R) nominee, that's about as far as it goes.  Trump is really more 3rd party than an (R).  Been that way from the moment he descended that escalator 2 years ago.  

Never in the modern history of US politics has a president been more opposed by such a large swath of "his own" party (even though it's often not prudent to declare it out loud).  If Trump decides not to run for reelection, million of R's will quietly cheer since we never wanted him as the nominee in the first place.