WASHINGTON — US political campaigns will spend $6 billion in federal races this year, making the 2012 general election the most expensive poll in American history, experts said Wednesday.

The presidential election pitting President Barack Obama against Republican challenger Mitt Romney, alone accounts for $2.6 billion, according to the Center for Responsive Politics, which analyzes US electoral expenses at opensecrets.org.

That's a decrease from the record-breaking 2008 campaign total of $2.8 billion, when Obama had to also finance a Democratic primary fight against Hillary Clinton.

The total includes spending by presidential candidates and major party committees -- about $2 billion -- $528 million from outside groups that report spending to federal authorities and $142 million from party convention host committees and public funding for the national conventions.

Overall, the 2012 election is set to top the last most expensive election -- four years ago -- by about $700 million, due in large part to an unprecedented amount of money of over $970 million raised and spent by outside groups following a decisive 2010 Supreme Court decision.

"In the new campaign finance landscape post-Citizens United, we're seeing historic spending levels spurred by outside groups dominated by a small number of individuals and organizations making exceptional contributions," said Center for Responsive Politics executive director Sheila Krumholz.

The court ruling allowed these groups to raise unlimited amounts of money from donors, and the organizations have been spending at a rapidly increasing rate.

Spending by such groups for and against Obama and Romney has jumped from $19 million per week in early September to $33 million per week in early October to $70 million during the week beginning October 21.

Conservative organizations -- including non-profit groups and so-called Super PACs political action committees -- largely dominate their liberal counterparts.

Crossroads GPS, founded by president George W. Bush's former right-hand man Karl Rove, has spent some $65 million to date.

Spending on congressional races in the Senate and House of Representatives was projected to increase slightly to $1.82 billion, up from $1.81 billion in 2010.