I've had discussions with Democrats who think that saying bad things about a political opponent during a campaign is dirty politics, even if what is said is true. For example, Al Gore was criticized for stretching the truth, even accused of some outright falsehoods, such as his previous voting record on abortion and his father's vote on the Civil Rights Act. Now why was it a bad thing to expose Al Gore's untruthfulness? Gore supporters claimed he was going to be a great President, but my response to them was "How do you know he is going to do what he says he is going to do? Do you even know if he intends to?" I mean really, in the end he was only telling people what they wanted to hear without even considering the costs or whether his goals were self-contradictory.
Now consider another way untruthfulness can corrupt a political campaign: How do you know that your untruthful candidate is telling the truth when it comes to public policy? Outright lies have been told about the environment, guns & crime, Social Security, Medicare, Iraq's possession of WMDs, how often third-trimester abortions have been performed, etc. If someone wants to vote intelligently on these issues, how can they do so based on what they hear from candidates who lie when they talk about these issues?
My point is that honesty and integrity are fundamental values, and deserve primary consideration before evaluating other issues.