House and Senate candidates have already shattered fundraising records for a midterm election and are on their way to surpassing $2 billion in spending for the first time, according to new campaign finance data.

To put it another way: That's the equivalent of about $4 million for every congressional seat up for grabs this year.

The frantic fundraising by candidates has largely been overshadowed in recent weeks by a tide of spending by outside interest groups, most of it targeting vulnerable Democrats. Such groups could spend $400 million or more by Nov. 2.

But the latest Federal Election Commission data, along with a new study from a campaign watchdog group, show that most of the money sloshing around the 2010 elections is being raised and spent by the candidates themselves.

As of last week, House and Senate campaigns reported taking in more than $1.5 billion, exceeding the total collected by congressional candidates in 2006 and in 2008, FEC data show. Most of that money already has been put toward advertising and other expenses.

The Public Campaign Action Fund, a watchdog group, will release a study Tuesday predicting that House candidates alone could spend nearly $1.5 billion by the time the dust settles on Election Day. The calculation is based on previous elections in which about half of a campaign's money was spent in the final month of the contest.

Senate campaigns are also on track to exceed the $550 million mark from 2006, bringing the likely total to $2 billion or more by the time the ballots are counted.

The surge is driven in part by the unusually broad battlefield in the House, where an estimated 90 seats are in play, almost all of them held by Democrats. Many Democratic incumbents are emptying their coffers in an attempt to win the message wars against GOP-allied interest groups.

"Both members of Congress and their challengers need to raise a huge amount of money to respond to these outside groups," said David Donnelly, national campaigns director for the Public Campaign Action Fund, which advocates for public financing of elections. "Candidates are losing control of their elections unless they get on the phone to raise money to get their own ads on the air."

Some of the most striking increases are evident on the House Republican side, where a deep bench of competitive candidates could wrest control of the chamber from Democrats. Republicans have also raised more and spent less, giving them an even larger advantage in the last week of the campaign. Through the third quarter of 2008, Democratic House candidates had outraised their opponents by $64 million. This year, the balance has been reversed, with Republicans outraising Democrats by $30 million, according to the action fund's analysis, which is based on data from the Center for Responsive Politics.

GOP political consultants say the reasons for the shift are simple: Republican voters are more enthusiastic and they are eager to give money to challengers seeking to oust Democrats.

"The congressional committee has recruited a lot of good candidates and the tea party has brought in a lot of candidates, and that means more places are in play," said Joe Gaylord, who was an aide to former House speaker Newt Gingrich (R-Ga.) and now heads Chesapeake Associates, a political consulting firm. "All of that means more dough."