Thompson was up 9 points in the polls a month ago, and now he's down 9 points. It's useless to fret about whether the Marquette poll is accurate: How inaccurate can it be? How did it happen? Tammy Baldwin ran negative ads against the once presumptively popular Tommy Thompson, and they obviously worked. Here's the one I've seen on TV most often:
That's an AFSCME Independent Expenditure ad, not Baldwin's own campaign. Here's AFSCME's YouTube page, where you can see the kind of specific, effective material they are putting out in many local campaigns. For example, here's one in the Ohio-16 district, using a very similar attack — a politician went to Washington and got allied with "special interests" — and similar — and better — use of animated graphics to present a simple, clear message.
Tommy must have been thinking that he could cruise to victory, but the old man is getting kicked around.
ADDED: Tommy's campaign tried to go negative, via email to various conservative contacts, just before she did her big DNC speech, but Tommy got burned. Tammy supporters were able to portray this material as lesbian-bashing, and, in fact, that meanness — more than the ads — may be at the root of Tommy's troubles.