I regard it as a given that all human beings tell lies, and that goes double for politicians. Then there is the problem of what is a "lie" and the question of whether all lies are equally reprehensible.
Folks say things that are not in accord with what is later determined to be some set of facts. Mistakes and misunderstandings that flow from not having full access to all facts are common. People say things that they truly believe, only later to find that they were wrong. Our individual prejudices, and we all have those whether we admit it or not, color our beliefs. That's being human, rather than some sort of logical rationalizing machine.
Intemperate and hyperbole during a heated political campaign without regard to the facts, or based on the flimsiest of facts, has been the norm in this country since the campaign of 1800. Anyone who believes anything said during these quadrennial affairs is either dim-witted or so dedicated to the Party-line that they are intentionally blind. It may be unfortunate, but the political game is played that way and any candidate that doesn't get down into the sty and wrestle with the pigs is likely to lose the election. Still, its a handy campaign tool to accuse the opposition of preferring lies to the truth while ignoring our own campaigns failings. The candidates, generally try to take the "high road" mouthing pleasant generalities and giving extravagant promises. They say as little as possible of substance, and leave it to their henchmen in the trenches to use brass knuckles, and innuendo.
Folks "spin" almost everything they assert because they don't want to offend, or they wish to spare the feelings of others. Using words laden down with emotional weights makes misunderstanding of intention easy. We all spin, but when a politician does it all of a sudden it becomes reprehensible. Politicians are supposed to keep their mouths shut, and say nothing that can be used by opponents against them. Both Sen. Biden and McCain are well-known for shooting from the hip without stopping to carefully consider the effects of their words. These guys, both of them, are passionate and sometimes let their emotional first response pop out of their mouths before thinking through all of the implications that might be drawn. They aren't "liars" in the same way that a person lies when committing perjury.
Perjury is an example of the reprehensible lie that as a society we tend to punish. That is knowingly and intentionally, under oath, representing known falsehoods as factual and true. People do that. Criminals do it all the time. Nixon was prepared to do that, and Clinton actually did commit perjury.
Personally, I don't believe that either Sen. Biden or Sen. McCain are guilty speaking reprehensible lies. They both speak out in ways that is open to misinterpretation, and sometimes the opposition doesn't have to work very hard to make their words seem terrible. These are both honorable men, men who have dedicated most of their adult lives to serving the nation to the best of their ability. I wish that they were not subjected to the mud slinging that is associated with Presidential campaigning, but thats the way it is. No ideal world, and no ideal candidates who aren't sullied in some manner.