With the Senate majority potentially in the balance, tens of millions of dollars have poured into more than a dozen states for the final push in an unusually large cluster of competitive races.

Each side committed several million dollars more to tight Senate campaigns that had been anticipated battlegrounds since last year, particularly Virginia, Wisconsin and Montana, but late movement in several races forced the hand of some groups to finance ads in states where the race had long ago seemed settled.

Nowhere is that more emblematic than Nebraska, the deep-red state where state Sen. Deb Fischer (R) had been holding a very large lead over former senator Bob Kerrey (D) in the race to succeed retiring Sen. Ben Nelson (D). Kerrey, however, has closed that gap somewhat by sharply criticizing Fischer’s lawsuit against her neighbors over a ranching dispute.

While Fischer is still favored, Crossroads GPS, a conservative group run by allies of Senate Minority Leader Mitch McConnell (R-Ky.), bought ad time in Nebraska to go after Kerrey to try to shore up what was once considered a sure thing for Republicans.

The Nebraska ads came as part of a massive 10-state, $10.5 million campaign for the final week by Crossroads GPS, a nonprofit that does not disclose its donors. Crossroads and its affiliate, American Crossroads, had an advertising budget of roughly $100 million for 2012 to benefit Senate Republican candidates, hoping to pick up four seats to deliver a GOP majority. The U.S. Chamber of Commerce, which will be on air in four states over the last two weeks of the campaign, has a similar-sized budget for Senate races.

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The Majority PAC, a super PAC run by allies of Senate Majority Leader Harry M. Reid (D-Nev.), has its own final flourish of outside money. In Montana, for example, where Sen. Jon Tester (D) is locked in a neck-and-neck race with Rep. Denny Rehberg (R), Majority PAC has purchased about $900,000 worth of television ads for the final two weeks of that race, according to an advertising tracking document provided by Republicans. Across the nation, Majority PAC and its affiliate nonprofits plan to spend $7 million on Senate races in the final week, according to aides.

One effect of the outside, unlimited cash has been to expand the political map at a time when, in past elections, it would now be narrowing. Crossroads and the Chamber have been advertising in three states where most experts believe the race is settled in the Democrats’ favor: Maine, New Mexico and Hawaii. With such unlimited cash, the outside groups can also provide defense when a candidate goes wobbly, helping the candidate regain his or her standing and allowing the party committee to reserve its cash for the most competitive races.

Just as Crossroads has poured money into Nebraska to defend Fischer, Majority PAC purchased more than $500,000 worth of ads attacking the GOP Senate candidate in Pennsylvania, Tom Smith, after Democratic Sen. Bob Casey’s campaign got caught off guard by the wealthy coal executive’s self-financed $17 million war chest.

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.

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.