With five weeks left before the 2018 midterm election, House and Senate fundraising arms have raised and spent more than a billion dollars in their efforts to win control of Congress. Much of that money is flowing to a relatively small number of the most competitive House races.
The flood of cash is upending contests in some once-solid Republican strongholds, including a handful of districts that traditionally have been the most conservative in the country.
About 40 of those races include members of the Freedom Caucus, a group of conservative Republicans elected in the so-called tea party wave a decade ago. Most of those incumbents are expected to easily win re-election.
But the GOP is facing tough challenges from a handful of Democrats who have, so far, outraised their Republican rivals, according to data from the Center for Responsive Politics.
Here are six races where Democrats are spending big to defeat GOP candidates endorsed by the Tea Party Patriots, the tea party's fundraising committee, or the Freedom Caucus:
This coastal Southern California district, one of the tightest races in the country, pits incumbent Republican Dana Rohrabacher against Democrat Harley Rouda, a lawyer and real estate businessman.
According to a recent New York Times/Siena College poll, the race is a dead heat, with both Rouda and Rohrabacher drawing 45 percent of the vote.
An affluent patch of Southern California, the 48th District has been a traditional Republican stronghold. The GOP holds a 10-point advantage in voter registration. But Hillary Clinton won the district by 2 points.
The most recent data show that political action committees have spent a scant $150,000 each. But Rouda has plowed some $1.2 million of his own money into the race while most of Rohrabacher's $1.7 million has come from large individual donations.
Rouda was endorsed by both progressive groups and the mainstream Democratic Congressional Campaign Committee in a contested primary. Rohrabacher voted to repeal and replace the Affordable Care Act, but voted against the tax reform bill.
This northeast Iowa district has been a GOP stronghold for incumbent Republican Rod Blum, who was first elected in 2014. But voters in the Iowa 1st District elected Barack Obama president twice by double digits. In 2016, Trump won the district by less than 4 points.
Blum, a software entrepreneur, is facing Democratic challenger Abby Finkenauer, 29, a member of the Iowa House of Representatives first elected to the statehouse in 2014. She's raised more than $3.7 million as of the latest filings, including heavy support from political action committees including the DCCC and Emily's List.
Finkenauer's fundraising appears to be paying off. She was leading Blum 52 percent to 37 percent, according to a recent New York Times Sienna College poll.
This race became competitive in July when incumbent Republican Robert Pettinger was knocked out of the running in the 2018 GOP primary by Mark Harris, a Baptist minister who has picked up the endorsement of the Tea Party Patriots.
Pettinger had raised and spent more than $6.5 million before losing to Harris, who has raised less than $1 million, based on the latest filing available.
Harris faces Democrat Dan McCready, a businessman and retired Marine, who has raised more than $2.6 million, mostly from large donations from individuals.
North Carolina's 9th District voted Republican by double digits in the last three presidential elections. But political pundits have rated this race a toss-up.
Incumbent Republican Ted Budd, who has also been endorsed by the Tea Party Patriots, has lagged his Democratic opponent, lawyer and businesswoman Kathy Manning. Much of her campaign cash has come from out-of-state donors.
As of the latest filings, Manning had raised nearly $2 million to Budd's $1.2 million, which included $560,000 in contributions from PACs including the Club for Growth.
Budd was elected in 2016 in this midstate North Carolina district, which voted Republican in the last three presidential elections. But as this race has tightened, political pundits are now calling it a toss-up.
This sprawling district, which covers the southern half of New Mexico, became an open race when incumbent Republican Steve Pearce, a Freedom caucus member, announced his run for governor.
GOP state Rep. Yvette Herrell won a four-way primary in June and picked up an endorsement from the House Freedom Caucus. She is running against Xochitl Torres Small, an attorney and former law clerk, who has picked up support from the DCCC and Emily's list.
As of the latest filings, Herrell had raised about $500,000, much of it spent during the primary. Torres Small had raised more than $900,000 with about $500,000 in cash on hand.
The district has voted for Republican candidates in the last three presidential elections, but political analysts are now calling this districts a toss-up.
The Tea Party Patriots have also endorsed GOP incumbent David Brat, an economics professor first elected 2014 after his upset victory in a GOP primary against then Majority Leader Eric Cantor.
Brat's Democratic opponent, Abigail Spanberger, is a former CIA operations officer and U.S. postal inspector. This suburban Richmond, Virginia, district has voted solidly Republican in the last three presidential elections. But heavy fundraising by Democrats has helped Spanberger pull even with Brat. They each have roughly $1.3 million, as of the latest fillings.
Political pundits are calling the district a toss-up. In the last few weeks, a Monmouth University poll had the race tied at 47 percent each, while Brat had a 4 point lead in a New York Times Sienna College poll.
Get the best of CNBC in your inbox