Brooks is not, nor has he ever been a Democrat.   The problem is that most people have forgotten what a Republican used to represent.  I don't recognize much of the Republican party today.    Brooks can most certainly be a Republican and not support the mentally unbalanced Trump.  His concern is for the nation, not just scoring points for a political party.

https://en.wikipedia.org/wiki/David_Brooks_(cultural_commentator)#Political_views

Read it and recognize your bias.   I could also reference any number of liberal web sites that blast Brooks for his conservative positions, in particular social conservatism.

Brooks is a very smart man, but unlike some, also has the quality of humility.  If you read the link, you can see that his mind is not Shut and through his life has changed his opinions as he get's new information.   That's a sign of intelligence not partisanship.