Millions pouring into tight Senate races

The final ad purchases also provide a framework for where the party committees and outside groups believe the Senate majority will be decided. The Democratic Senatorial Campaign Committee, the official political arm, which takes limited donations, is advertising in 10 states, according to aides. Seven of those are held by Democrats: Connecticut, Virginia, Ohio, Wisconsin, Missouri, Montana and North Dakota. Three are held by Republicans: Indiana, Nevada and Arizona.

In addition, the DSCC is spending heavily in Massachusetts, just not on TV advertising, because of a pact worked out between Sen. Scott Brown (R-Mass.) and Harvard law professor Elizabeth Warren (D) to keep out outside advertising.

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Democratic and Republican officials believe those 11 races will decide the majority, and Republicans will need to take eight of the 11 to reach 51 seats; if GOP nominee Mitt Romney wins the presidency, Senate Republicans would need seven of 11 to claim a 50-50 majority based on the tie-breaking vote cast by Romney’s running mate, Rep. Paul Ryan (R-Wis.).

The DSCC’s counterpart, the National Republican Senatorial Committee, is advertising in seven states for the final week: Virginia, Wisconsin, North Dakota, Montana, Nevada, Indiana and Arizona, according to aides. Another state or two, such as Ohio, could be added for the final days.

These outside groups are often the source of the most heated attack commercials at a time when the candidates themselves traditionally close with softer, more positive messages. The DSCC, for instance, launched an ad Tuesday accusing Rep. Jeff Flake (R-Ariz.), in a close battle with former surgeon general Richard Carmona (D), of voting to “let insurance companies kick women out of the hospital on the same day” of their breast cancer surgery.

Flake has responded with an ad showing Sens. John McCain (R-Ariz.) and Jon Kyl (R-Ariz.), whose decision to retire opened up the seat, saying that Carmona was “shameful” and “lacks integrity” because of his advertising campaign using their words of praise for his confirmation 10 years ago.

Perhaps no state has been so consumed by outside spending as Montana, where advertising is very cheap compared to large states with urban populations such as Ohio and Florida. There, according to the GOP tracking document, Tester ($311,000) and Rehberg ($515,000) will blanket the state’s seven small media markets with their closing ads in their competitive race.

However, outside groups on both sides will drop a combined $3 million onto the TV and radio airwaves in the final days of the race, far outpacing the actual candidates.

Three liberal groups — the DSCC, Majority PAC and Montana Hunters and Anglers — will all spend more than Tester in the final week. On the right, the NRSC and Crossroads will spend more than Rehberg’s campaign.

Over the entire election, Tester and Rehberg will have spent a combined $5.3 million on advertising.

The outside groups dwarfed that sum, with another $20 million worth of advertising for a race that has remained a dead heat since Rehberg entered it in early 2011.

This article has been updated since it was first published.