I like to take people at their word when it comes to their motivations and assumptions. This is probably because I've spent so much time having my own motivations questioned, or dealing with people ascribing false motivations to the politicians I agree with. For example, "Obama hates America," or "Democrats support (insert any liberal policy) because they just want to control people's lives." Or, "Obama wants to take away your guns in order to pave the way for a dictatorship." It's infuriating when it's done to you, so I try to avoid doing it to others.
I do like to attack assumptions that are clearly baseless though, or at least get people to consider alternatives. Like I might ask, "If you knew that liberals want stricter gun control because they honestly think it would reduce murder rates, what would you say to them?" Or, "If liberals only want universal healthcare to control people's lives, why do the countries at the top of the freedom index all have universal healthcare systems?"
When it comes to abortion, most pro-lifers will tell you they believe abortion is murder. There are definitely some who have puritanical anti-sex attitudes as well, but there are filtering questions you can ask to identify these people. Like, "would you support easier contraceptive access to prevent unplanned pregnancies?" And if the answer is no, I'd ask why not.
Until I have a clear reason to do so, I really hesitate to "psychologize" anyone in a debate, just because of its potential to poison the well.