These 10 critical Senate races can expect a surge of campaign cash in the next 5 weeks

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.

The pace of spending is on track to set a record, in part because of recent changes in campaign finance rules that allow for unlimited spending by hundreds of political groups and a handful of wealthy individuals. This year's midterms also come as the political divide has increased over a wide range of issues, from immigration reforms to the divisive Supreme Court nomination of Judge Brett Kavanaugh.

The stakes are higher this cycle because Democrats stand a good chance of winning back control of the House of Representatives, offering the party a check on Republican control of both Congress and the White House. Though political analysts give Democrats longer odds in winning back the Senate, the margin is razor thin. It would take a net gain of just two seats to flip control in their favor.

That thin political margin is reflected in the 10 races in battleground states that political analysts see as being too close to call. They include two open seats in Arizona and Tennessee, where incumbent Republicans Jeff Flake and Bob Corker are retiring, along with toss-up races held by incumbent Democrats Joe Manchin of West Virginia, Heidi Heitkamp of North Dakota, Clare McCaskill of Missouri, Jon Tester of Montana, Joe Connelly of Indiana and Bill Nelson of Florida. Democrats are hoping to win back seats held by Republican incumbents Dean Heller of Nevada and Ted Cruz of Texas.

Voters in those states have already endured a barrage of campaign ads, both for and against the candidates in those races. And they can expect even more spending in the coming five weeks before Election Day.

Much of the spending is coming from the major party committees that have raised tens of millions of dollars by bundling donations from wealthy individuals and companies into massive war chests. Just four committees — the Congressional Leadership Fund and Senate Leadership Fund for the Republicans, and the House Majority PAC and Senate Majority PAC for the Democrats — have spent more than $130 million on House and Senate campaigns, as of the latest data available.

But these four funds have raised more than $310 million, with more money continuing to pour in as the campaign enters its final weeks. As in past cycles, the pace of fresh cash coming in will likely accelerate as Election Day approaches.

The flood of campaign finance cash will likely remain concentrated in those 10 key races, where both sides see their best chances of swaying undecided and independent voters in the final weeks.

Here's how the spending race is shaping up in those 10 key races, as of the latest filings at the end of September.