I found a nifty tool that lets you view Senate voting records and thought I'd compare the "Not Voting" rates of Senators Kerry and Edwards for this Congressional term. Not surprisingly, both have "Not Voting" rates above 40% if the entire term is taken into account, but there's a difference to be seen if the timeframe is changed.
Both Kerry and Edwards announced their candidacy near the beginning of September, 2003, so let's only count votes before then. From January, 2003, to August, 2003, Senator Edwards didn't vote 69 out of 320 opportunities (~22%) and Senator Kerry didn't vote 182 out of 320 opportunities (~57%). Strangely, it appears that Senator Kerry started voting more after he announced his candidacy than he had before!
For comparison, other senators missed far fewer votes this term. For the Democrats, Senator Kennedy missed ~4% of the votes, and Senator Feinstein missed only ~2%. For the Republicans, Senator McCain missed only ~1%, and Senator Stevens missed even less than 1%. I selected these senators at random from the ones I knew off the top of my head, and I'm not trying to make a point about the parties but rather about the Senators running for the presidency.