If your supervisor insists that you take a company letter, there really isn't much that you can do. Just be polite...and, once again, ask your supervisor to show you some respect and not to bother you with these letters. You don't really have to read them or discuss them with him. 

You might want to ask "Why?" are they bothering you so much with these letters? "Why?" are they interfering so much with your work? And, "Who?" is really writing them? You might want to ask your supervisor if he is really comfortable bothering you like this. 

Challenge their facts! Don't let their lies, distortions and misrepresentations go unchallenged. Correct their distortions publicly! Propose that the union staff be invited to address these claims at a company meeting or at the captive audience meetings. Every opportunity you get, propose "open forums" where representatives of both the union and management can respond to questions of employees who voluntarily attend.