Social Identity Theory
Social identity is your perceived identity as part of a social group, like the LibDems. Current research suggests all sorts of interesting facts about your relationship to the political party you choose. For instance, you are more positive towards other members of the party, considering them better than people in other groups. You generally regard your group members as more attractive -- perhaps that's why many political marriages are between party members and not, for example, between Labour and Conservative. Mind you, by this criterion, same-sex marriages should be more common than they are, so judge this theory as you think fit.
You probably feel that membership of your chosen political party enhances your social position or lifestyle. But perhaps it doesn't and you feel put-upon and unhappy at how things have worked out. My psychological textbook states, "people accept a group's inferior situation if they believe their position is just and legitimate." For LibDems, this is undoubtedly true. Otherwise, why would so many LibDems put up with what we do from our Coalition partners. Also, people have a tendency to change groups if their group membership is perceived to be negative -- they will move to a group with a perceived higher status -- the "rats leaving the sinking ship" scenario. It explains why LibDem political party membership is falling. Then again, it's interesting to note that all political parties have declined in membership in recent years, so perhaps it's politics that's at fault. Politics as a whole is viewed negatively in society and people prefer to go gardening or call themselves magicians and go audition on Britain's Got Talent.
Do people discriminate against a particular political party? It's true in other areas of society. Well, I'm sure we all find elements of this in our behaviour, but at least the LibDems tried to put fairness and freedom at the head of their agenda. Do you discriminate against someone because of their sexuality or their gender? Probably not. But do you feel a nagging sense of superiority when someone says they're a Sun reader, or they try to disguise their incipient baldness with a combover? It's all prejudice, folks -- you see a member of another group, whether it be readers on a certain newspaper or bald people and you feel that you're better than them for whatever reason. A textbook definition of prejudice, and when you let that affect your behaviour, it becomes discrimination.
Finally, if we do feel fear or negativity because of a perceived inferiority, it is our natural inclination to search for allies to help us fight against the odds. Coalition anyone?