(CNN) The 2018 midterm elections are on pace to be the most expensive in history, with record-smashing fundraising fueling a geyser of campaign spending. At the forefront of the battle for Congress, and in particular the US House of Representatives, are the various national party committees -- the Republican National Comittee (RNC), the National Republican Congressional Committee (NRCC), the Democratic National Committee (DNC), and the Democratic Congressional Campaign Committee (DCCC).

Those four committees combined to raise nearly $70 million in just September, and data from the latest FEC reports sheds light on the states and zip codes powering the fundraising bonanza. The CNN analysis did not include the two Senate-focused party committees, since only a third of senators are up for re-election this year

California and New York, the first and fourth most populous US states, dominate the field for both parties, due to their unparalleled size and wealth, though the states' politics gives Democrats a significant edge in both. Republicans mostly make up for that disadvantage with a big edge in Texas, and a slighter edge in Florida and Nevada.

In September, the DNC and DCCC received by far the most money from zip codes in California and New York, preponderant even given their size. Of the top 25 zip codes ranked by total contributions, nearly all cover various parts of big cities in the biggest blue states: twelve zip codes from California, all from areas around Los Angeles, San Francisco and San Diego; four zip codes from New York City; and three others from parts of Chicago, Illinois.

But the zip code giving the most money to the Democratic committees: 20815, which includes parts of Silver Spring and Chevy Chase in Maryland, just outside of DC. Donors from that zip code have given nearly $400,000 to the Democratic committees. And it's an area that's been far less generous to Republicans, ranking just 1,651st on the list of zip codes giving to the RNC and NRCC, to the tune of just $1,405. A total of three DC-area zip codes were among the top 25 for Democrats.

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