It's not just "forgetting" that he had to report the use of a limousine and driver as income, or even finding out about his "obligation" in June and not doing anything about until he had the HHS nomination. Tom Daschle, as this post from the invaluable Glenn Greenwald points out, is a first-class money-grubbing opportunist and sleazebag -- one of many who went to Washington to "do good" and ended up doing very well indeed, for himself. Since being defeated for his senate seat in 2004 he has been working in the lobbying and "legal" vineyards and raking in the bucks. It's not clear just how much money he has made -- $5 million over five years, $4 million last year alone; the stories aren't specific. But he's been paid plenty by the healthcare industry he's supposed to "reform." Now that he's got the bucks he wants power again.
Furthermore, while he was in the senate his wife worked as a lobbyist for the airline industry. Somehow people who had business or issues before the senate were eager to hire her (and help paty his mortgage. You won't hear much about it because plenty of "power couples" in Washington, from both parties, play the same sleazy games.