WASHINGTON D.C. (WTAQ) - The Thompson-Baldwin race has officially become the most expensive U.S. Senate campaign in Wisconsin history.

Total spending has reached $40 million – topping the nearly $34 million spent two years ago when Republican Ron Johnson unseated 18-year Senate incumbent Russ Feingold.

The Federal Election Commission says outside groups have spent almost $28 million on the Wisconsin Senate race. And Republican Tommy Thompson and Democrat Tammy Baldwin have spent over $12 million of their own funds as of the end of September.

Baldwin was outspending Thompson by $4.5 million at the start of this month. But the Center for Responsive Politics says groups that oppose Baldwin have boosted their spending by about $4 million since the beginning of October, as Thompson gained in the polls.

Crossroads GPS, a group created by Republican strategist Karl Rove, has been the biggest outside spender at $4.6 million. The Democratic Senatorial Campaign Committee has spent almost $4.5 million to promote Baldwin and attack Thompson.

Federal officials say outside groups continue to spend more on the Wisconsin race than in every other Senate contest but one – Virginia.