HELENA, Mont. (AP) — Outside groups have already spent millions of dollars on ads attacking either Democratic Sen. Jon Tester or Republican Rep. Denny Rehberg in Montana's high-profile Senate race.

Lee Newspapers of Montana reports that three groups have spent more than $1.5 million on broadcast advertisements that go after Tester. Three other groups have spent $720,000 on anti-Rehberg ads.

Various other groups have spent between $800,000 to $1 million on ads attacking or supporting either candidate.

Almost all of the ads are sponsored by nonprofit groups that do not have to reveal who finances them.

The ads are known as "issue-advocacy" spots, which don't say vote "for" or "against" a candidate.

They instead mention the candidate's name, usually in a negative way, in connection with an issue.