Find a scratch piece of 8.5x11 paper, turn it lengthwise, and try to fold it on a 4:1 ratio (the crease should be 2.2 inches from the edge of the paper, you can check with a ruler after you make the fold). Now, take a new piece of paper and fold it in half. It's a lot easier, right? Just like your brain finds it easier to see the center than 2.2 inches from the edge, it's more convenient to find balance than to pinpoint how far from center something really is.
While the above example deals with spatial reasoning, the same is true for cognitive evaluation; when we say we want to hear 'both sides of the issue' we imply that we are going to apply equal weight to both sides, regardless of how crazy one side might be.
In American politics, you can get your way by winning the center or making the center come to you. Republicans have found that the latter is easier -- they simply move the center by staking absolutist right-wing positions, and then wait for the media's tendency to give both sides of any issue equal weight. Consider the way the debates on gun control and tax policy have shifted; the NRA and Grover Norquist have successfully taken any discussion of centrist, let alone liberal, gun or tax policy off the table... and over time their positions became 'conservative' rather than 'extreme.'
This unconscious leveling of the playing field that goes on during policy evaluation also occurs when we perceive leaders. Just by the nature of there being a two-way contest, we tend to see equality in competence where it shouldn't exist (see the 2008 Vice Presidential debate). Our inability to fold on a 4:1 ratio, to separate the electoral wheat from the chaff, leads to our inability to separate jokers from real leaders in general elections.
Democrats, and many independents, have so far found the Republican primary campaign hilarious. From Rick Perry's 'oops' debacle about cutting government agencies to Mitt Romney's 'I love lamp' moment about the state of Michigan, there has been plenty to laugh about. Looking past the gaffes, there are a number of actual policies advocated by the current field (like Newt Gingrich's moon colony) that are equally laughable. Together, these have culminated in the recent attempt to get Democrats to vote in the Republican primaries. Dubbed 'Operation Hilarity,' it alludes to the copious amounts of new material the candidates have generated for Jon Stewart. But the humor in this year's GOP race is dangerous -- it unconsciously moves the goal posts as to what is and isn't "presidential."
Any time you hear someone invoke 'lowered expectations' for a candidate remind them of the trap they are falling into. We should never have to lower our expectations, or shift our opinions, in order to accommodate a bad politician; our political class should be producing better leaders for us to choose from. And that's no joke.