BOSTON -- The U.S. Senate race in Massachusetts is not only one of the most closely watched in the nation, it's also turning out to be the most expensive.

Campaign donations in the contest between GOP incumbent Sen. Scott Brown and his chief Democratic rival, Elizabeth Warren, have already topped $30 million, with Election Day still more than six months away.

A review of Federal Election Commission records by The Associated Press found the next most expensive race was in Texas, which includes a crowded Republican primary. Donations in the Texas race have neared $28.5 million.

The state with the third most expensive race in terms of total donations is Florida, where candidates have collectively pulled in more than $17 million.

The review found that Warren, who has received $15.8 million since formally entering the race in November, has raised more than any other Senate candidate in the country.

Brown, who is defending the seat he won in a special election in 2010, has collected nearly $12 million in donations during the current election cycle.

Several other Democratic candidates in Massachusetts have raised smaller sums, although the only one remaining in the Senate race is Marisa DeFranco, an immigration attorney from Middleton, who has raised nearly $42,000.

One reason for the large sums being collected in Massachusetts is the highly competitive nature of the race.

Recent polls have swung back and forth between the two candidates and both national parties are banking on winning the seat. The Democrats want to hold on to their narrow majority in the Senate, and Republicans are hoping to wrest that control away.

Another reason for the fundraising surge in Massachusetts is a pledge that Brown and Warren both signed designed to limit advertising by third-party groups.

Under the deal, whichever candidate benefits from a third-party ad must write a check for half the ad's value to a charity named by the other candidate.

Brown has already written two checks for more than $35,000 after outside groups ran ads on his behalf.

The absence of third-party groups pouring millions into the campaign means both candidates will have to rely instead on their own campaign money to get their message out to potential voters.

That has led to nearly daily fundraising appeals by email from both camps, which routinely portray each other in the harshest light possible. Each attack by one candidate inevitably inspires a campaign fundraising appeal by the opposing candidate.

It's a contrast from other political contests in the wake of the U.S. Supreme Court's 2010 ruling in the Citizens United case, which has made it easier for corporations and wealthy donors to pour millions into campaigns.

Both campaigns have tried to use campaign fundraising records against each other.

Brown's campaign points out that Warren has collected the bulk of her money from outside of Massachusetts, including donations from Hollywood heavyweights like Barbra Streisand, Danny DeVito and Dreamworks CEO Jeffrey Katzenberg.

Warren's campaign has tried to portray Brown as too cozy with Wall Street.

Warren's campaign also notes that in the 2010 special election to fill the seat left vacant by the death of longtime Democratic Sen. Edward Kennedy, Brown benefitted from a flood of out-of-state contributions from donors who supported his pledge to a critical vote in the Senate against President Barack Obama's health care overhaul.

An earlier AP review of campaign finance records found Brown was relying heavily on donations from the financial services and health care sectors, while Warren was tapping the wallets of lawyers, fellow academics, union members and filmmakers.

Brown has also collected about five times as much as Warren from political action committees.

Other Senate race where total donations have topped $14 million, according to Federal Election Commission records, include: Arizona, Connecticut, Michigan, Missouri, Montana, New Jersey, New York, Ohio, Pennsylvania, Virginia.

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