The 2014 midterms will be the most costly congressional races in U.S. history, on track to cost nearly $4 billion, according to a new projection by the Center for Responsive Politics.

That would surpass the record-setting $3.6 billion spending on House and Senate elections in 2010 and 2012. Including spending on the presidential race, the 2012 election cost a record $6.2 billion.

This year’s surge is being powered largely by independent political groups such as super PACs and nonprofits, which are expected to dump nearly $900 million on messages to voters on the air and the ground.

Candidates are expected to spend $1.66 billion in all, down from the $1.79 billion they spent in 2010. Republican candidates are on track to have the spending advantage by a margin of nearly 57 percent to 43 percent.

And overall, the center projects that the GOP will have a slight advantage in the money race, with its party committees, candidates and allied groups spending at least $1.92 billion combined to the left’s $1.76 billion.

The figures only include campaign expenditures reported to the Federal Election Commission, so the totals do not reflect the tens of millions of dollars spent on "issue ads" by tax-exempt advocacy groups.