"The other members of your union shop would surely demonstrate the long-held and well-documented labor union policy of free and independent political discourse and so would fully support and celebrate your taking a day off to attend a Trump rally", the thought bubble above my head does not read.

Openly support someone who is anti union and see how much they protect your political opinion

YOU.

#44 wow. what did this Glenn person do to YOU?

It doesn't really matter where the coworker said OP went on their day off. The point is that the boss thinks OP lied about being out sick. If OP has a doctors note, a recent prescription for a short term illness or any other proof they were actually sick- they can easily get this turned around. But without proof, it's OP's word against the coworker's. I was in OP's situation once (except that my dickbag of a coworker claimed they saw me at the movies, not a rally). The only thing that saved my job was the fact that I still had a copay receipt from my doctor's office, dated the same day I was allegedly out goofing off at the theater.

They never said it was a sick day.

Unless they took the day off because they were sick or had an emergency, and their boss thinks they lied.

Doesn't matter unless they work in an at will state like California you cannot be fired for taking a day off, especially one that was approved, this has wrongful termination lawsuit all over it.