I do not believe they are allowed to do this.
I know that active-duty military aren't allowed to do this. First, they aren't supposed to be at political events in uniform (or using their military status). Second, if they bash the President publicly, they are guilty of violating Article 88 of the UCMJ, Contempt towards officials.
Thanks SIDENET! I was posting as you were sending. Nice to have the UCMJ Article number.