While there's always going to be a certain debate about where the line falls between eccentricity, non-conformity, and actual mental instability, I've always thought that a useful working definition is to ask, "Is this person acknowledging reality?" That is to say, do their beliefs actually coincide with the facts? A mentally ill person might believe that all dogs are secretly surveillance robots there to spy on him. This is provably untrue; ergo, this person is not sane.
The direction political debate has taken in the United States leans towards insanity. As with so many things in life, it can all be summed up in a Doctor Who quote: "The very powerful and the very stupid have one thing in common. They don't adjust their views to fit the facts. They adjust the facts to fit their views." As of late, people have adjusted the facts to fit their views.
Go onto any set of right-wing blogs, read a description of an event, trend, or movement. Go onto the left-wing blogs. Read the same descriptions. They are not describing the same things. It's no longer a question of interpretation--they literally do not apprehend the same reality as one another. Debate cannot continue when neither side acknowledges that the other side is accurate in basic factual contentions, let alone their views on how those facts should be interpreted. It's hard enough deciding what color to paint the house when we can't even agree on what "blue" is, let alone whether or not it's a nice color.
It's an extremely tricky problem to fix, too, because each side thinks the other has the problem. I think it's the right. I think they've been systematically devaluing the very concept of "fact" because all too often, the facts don't support their viewpoint (evolution, poverty, the economy, weapons of mass destruction, who declared a state of emergency when and where in Louisiana, going all the way back to "how many votes did the Republicans get in Florida?") But I can't be sure, because I'm on the wrong side of the viewpoint gap to see whether or not the left is distorting things.
But it's something that has to change. The news media have to be more cautious in fact-checking (especially in checking politicians' speeches--if they lie, the media have to catch it quick) at every level, from CNN down to individual blogs. Facts are too important to be subverted to ideology. There's never been a situation where you can ignore a fact to support a belief and have it not come back to bite you someday. That's why we call these people "insane" and not merely "eccentric" or "non-conformist". Because while other people might be willing to bend to accomodate your eccentricities, reality doesn't.