If you live in a state being bombarded with Senate campaign ads, get ready for an all-out assault this month.

Political action committees, or PACs, have raised more than $790 million for the upcoming midterm election. But only about a third of that money has been spent on behalf of congressional candidates so far, according to a CNBC analysis of campaign finance data.

And though there are 35 Senate seats up for re-election, the bulk of so-called outside spending is being deployed in just 10 of the most competitive races.

With just five weeks left to the crucial November election, PACs are flooding the airwaves and Web sites with mostly attack ads in a battle for control of the Senate. There's no limit to how much these so-called "soft money" groups can spend as long as they don't coordinate their spending with individual candidates.