It's still early in the midterm cycle, but long-distance donors are already flooding tight House races with cash

• In many of the most competitive House districts, money is flowing in from outside donors.
• Democrats are hoping to ride the wave of anti-administration sentiment that has beset the party in the White House in other recent midterm cycles.
• This time around, to win back the House, Democrats will need to flip 23 seats held by Republicans.

Adam Jeffery | CNBC

The U.S. Capitol Building in Washington, D.C.

One of the most the most important midterm elections in decades is still more than seven months away – but big money campaign donors are already lining up to do battle for control of Congress.

In many of the most competitive districts, money is flowing in from around the country. Both parties have worked up lists targeting key districts they think are vulnerable.

Democrats are hoping to ride the wave of anti-administration sentiment that has beset the party in the White House in other recent midterm cycles. To win back the House, Democrats, who are eager to exploit President Donald Trump's low approval ratings, will need to flip 23 seats held by Republicans. The GOP is hoping to hold their majorities in both the Senate and House.

While all 435 House seats are technically in play, only about 50 are considered truly competitive. Many of those are in the more than 30 districts where Republicans have said they are not seeking re-election, a figure that is higher than in past cycles. Democrats will have to defend 16 open seats.

While the money battle is heating up, it's still relatively early in the current campaign fundraising cycle. Still, some states and districts are already seeing large flows of money heading their way from donors outside the district. As of the latest campaign finance filings, about two-thirds of the roughly $750 million in individual contributions to House candidates has come from donors outside that candidate's district.

Moreover, in 40 of the most competitive House races, more than three times as much of those individual donations are coming from outside donors than from voters who live within the district's boundaries. That's a strong sign that large, nationally focused donors see those races as critical to winning control of Congress.

In some cases, the money from outsiders has been raised by an incumbent who is not running for re-election. The biggest such war chest, more than $10 million, has been amassed by House Speaker Paul Ryan of Wisconsin. It remains to be seen where that money will flow now that Ryan has announced he won't be running for re-election in November.

Tracking the money

To better analyze the impact of these long distance donors on this year's midterm elections, CNBC has partnered with the Center for Responsive Politics to track where the cash is flowing. As campaign finance has become increasing concentrated among a relatively few individual donors and committees, these funding sources have taken on an outside influence on the outcome of the American elections.

Still, tracking and analyzing the flow of money in American politics is fraught with pitfalls and roadblocks created by the reporting system set up by Congress and later shaped by court rulings that have created loopholes for donors willing to take the trouble to try to avoid reporting where their money is going. Following a series of federal court rulings in 2010, so-called super PACs are now able to raise unlimited amounts from individuals, industries, unions, corporations and other groups and make "independent expenditures" targeting a specific issue, party or candidate.

In some cases, the source of campaign funding isn't reported at all. This so-called "dark money" is raised by nonprofit, tax-exempt groups that don't have to disclose where their funds come from as long as they don't coordinate their activities with a specific candidate or party committee.

Despite the increasing influence of this loosely reported spending, the bulk of the money financing this year's midterm elections comes from individuals, companies, and organizations that disclose their donations. Some of that money comes from small contributions that are not itemized, so it's impossible to determine whether the donor was a constituent in a given district or an outside party trying to influence the outcome.

But most of the money can be tracked. Of the roughly $1.2 billion in direct contributions raised so far, only about $190 million represent contributions from individuals who have not been itemized with geographic data. That provides a window into where the money is coming from and which races are drawing the most attention from outsiders

Here are five key races that have drawn outsized campaign contributions from long distance donors who won't be voting in the districts they're targeting.

California 39th

As in Ryan's case, the incumbent, Republican Ed Royce, who is not running for re-election, has collected the biggest pot of long-distance donations in this race. Before announcing he was bowing out, Royce raised some $1.2 million from outside his Southern California district, which has attracted a dozen candidates.

Democratic challengers have far out-raising the Republicans vying to hold this seat. Of the money that can be tracked by geography in this race, the bulk of it has come from outside donors, much of it from the nearby Los Angeles metro area, a major source of long-distance donations across the country.

So far, despite the heavy flow of money, a recent poll showed the race remains wide open, with more than a third of voters undecided.

New York 19

In some case, an otherwise "safe" seat is being put in play by the flood of money coming from the opposing party.

That's what's shaping up in New York's 19th district, where seven Democratic candidates –including Antonio Delgado, a corporate attorney, and Pat Ryan, a local business owner – are jockeying to face first-term Republican congressman John Faso. Roughly three-quarters of the itemized donations in this race so far have come from donors living outside this mid-state district. More than half that money came from the New York city metro area, another big pool of long distance donations.

Virginia 10th

In suburban Washington, D.C., two-term incumbent Republican Barbara Comstock has raised nearly $2.8 million, with about 85 percent of itemized contributions coming from from outsiders, Many of them are located in the nation's capitol, another big source of long-distance campaign finance.

So far, Comstock's leading Democratic fundraising rival, former State Department official and political activist Alison Kiehl Friedman, has collected nearly $1.4 million in direct contributions, with more than $800,000 coming in from donors as far away as California.

California 49th

After nine terms in the House, Republican Darrell Issa's decision earlier this year not to run for re-election has raised Democrats' hopes of flipping this district north of San Diego, which favored Hillary Clinton over Donald Trump by more than 7 points in the 2016 presidential election.

The district is one of seven Republican-held seats that voted for Clinton in California, a state with an "open" primary system that sends the top two vote-getters to the general election, regardless of party affiliation. That means in districts like the California 49th, voters could end with two candidates from the same party facing off for the same seat in November.

So far, Democrats have raised nearly $6 million as a group, more than four times as much as their Republican opponents. Much of the money raised from outside the district is coming from within California.

Michigan 11th

Michigan's 11th congressional district is another open race, following the announcement last fall that incumbent second-term Republican Dave Trott would not run for re-election. Since then, five Democrats and five Republicans have entered the race.

Republican businesswoman Lena Epstein leads in fundraising, having collected nearly $1.5 million, with more than $1 million in cash on hand. Only about a third of those donations that can be tracked by geography have come from outside of her district.

On the other hand, her Democratic rivals have raised more than twice that amount between them, with the bulk of those itemized donations coming from outside the district.