it's very easy to paint a senators voting record as negative. Clinton has tried in several debates, and Obama has done a good job of rebutting her claims. I think specifically of the debate where she put into question how many times Obama cast a "non-vote" in the Illinois senate, trying to frame him as someone who was either non-committal, absent, etc. He rebutted with the fact that if there was a good bill with some questionable aspects, he felt it best to non-vote, as he didn't want to look like he was against the principle of the bill, but also couldn't in good faith allow it through with the questionable portions intact