Well, politics runs on two fairly simple propositions, Emphasize your favorite candidate's strong/good points and emphasize only the weak/bad points about the opposition.
The MO of Rovian-type, winning-is-all-that-matters-at-any-cost, politics is to just make up for any lack of material by just making it up. So if they are making up the bad stuff about the opposition one must also wonder about how much of the good stuff of their candidate is likewise made up. (Hindsight may now be giving us a fairly clear 20/20 perspective on the value/merit of Rovian political "success"...)
Now on the other hand, when there is plenty of true strong/good stuff about your candidate there isn't much need to build him up by tearing down the opposition. And furthermore, the plentiful true bad facts and weak points about the opposition mean that there there is no need to make things up on that side of the equation either.
(Have you wondered why one side seems to obsessively feel a need to assure us that they hold the moral high ground... Hmmm...

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And that is why you see an obvious imbalance in spreading of lies. And it has absolutely nothing whatsoever to do with the relative moral/ethical value placed on honesty and truthfulness by either party... Oh not at all!

Tom