1) They are not really your friends.
2) They are using a version of the "everyone does it" defense to protect Bill and Hillary.
3) To Libs, it is not whether Reagan covered up or participated in Iran-Contra, it is the seriousness of the charge
4) 2 and 3 are done to silence you, to get you stammering, to choke the message that you want to deliver- do not let them silence you.
Take whatever tact you want to, but, IMHO, you should remind them that Bill Clinton is a monster. He raped woman, sold our most precious secrets for next to nothing, lied under oath, abused his powers and sentenced to death a man who was mentally incompetent to stand trial, as governor of Arkansas. Then ask them how Billy boy compares to Ronald Reagan.