(CNN) Republican operatives working to hold onto the party's House majority are increasingly worried about the so-called "green wave" of money flowing to Democratic candidates in the final weeks before the midterms.

Fundraising reports from the third quarter of 2018 cemented those fears as Democrats frequently outraised their Republican opponents in competitive House races.

The advantage has boosted Democrats in key House races in a variety of races, like Andy Kim in Central New Jersey, Jared Golden in Northern Maine, and Sean Casten in the Chicago suburbs, all of whom raised over $2 million in their races. Candidates like Kim Schrier east of Seattle, Josh Harder in California's Central Valley, Amy McGrath in central Kentucky, and Katie Hill north of Los Angeles all raised over $3 million -- massive hauls for candidates, some of whom likely won't be able to spend all that money in inexpensive television markets.

Campaigns with larger fundraising hauls are able to spend more on television and digitals ads and build out their get-out-the-vote operations, two key factors in deciding races.

Eye-popping numbers like those will keep Republicans up at night through Election Day, operatives said, with one Republican strategist bluntly saying that Democrats' ability to raise money and outspend Republicans in the closing weeks was their "biggest fear."

Those fears are exacerbated when Republican incumbents fail to keep up. Abigail Spanberger, the Democrat running to oust Rep. David Brat in Central Virginia, more than tripled the Republican incumbent's third quarter take, bringing in $3.6 million compared to $1 million for Brat. In the Houston suburbs, challenger Lizzie Fletcher more than doubled Rep. John Culberson's take, $2.2 million to $800,000.

Fundraising gaps like these have led Democratic candidates to reserve millions more in advertising through the end of the election. According to data from CMAG/Kantar Media, House Republican candidates have $38.7 million in ad reservations through the November 6 election, compared to $60.1 million for Democrats.

Matt Gorman, spokesman for the National Republican Congressional Committee, said that the committee has long told Republican lawmakers and candidates that "being massively outspent in your race is a simply untenable position."

"You must not only have enough money to define your opponent; you also need enough to define yourself," said Gorman, pointing to the Rep. Conor Lamb's victory earlier this year, where he outspent Republican Rick Saccone by a 3-to-1 margin.

In total, according to the Democratic Congressional Campaign Committee, eight Democratic candidates raised over $3 million in the third quarter of 2018, a staggering figure that illustrates how Democratic excitement at defeating Republicans in November has turned into a substantial money advantage.

Rep. Ben Ray Luján, the chair of the Democratic committee, called the fundraising numbers "unprecedented" and argued that they are a representation of "of energy and momentum" on the Democratic side.

Both Republicans and Democrats have been boosted by considerable outside spending, with the Congressional Leadership Fund, the primary super PAC backed by House leadership, announcing earlier this month that they have raised $132 million this cycle, and House Majority PAC, the corresponding Democratic outfit, has raised $51.4 million in 2017 and 2018. Former New York Mayor Michael Bloomberg, who is flirting with a run for president in 2020 as a Democrat, has also pledged to donate $80 million to Democratic causes in 2018, including substantial contributions to the fight to take back the House.

Republicans, as was the case in part cycles, continue to maintain an edge in outside spending on television. Republican outside groups are spending $111.4 million on TV and radio ads (including future ad reservations) in 2018, and are set to spent nearly $70 million in just the final month of the election, according to date from CMAG. Democratic outside groups are spending nearly $80 million on TV in 2018, with nearly $60 million coming in October and November.

That money, however, only goes so far.

Federal law mandates that political campaigns receive discounted rates for television ads, making money spent by a candidate far more efficient than that given to a super PAC, which has to pay the full rate to run ads. That means that in some districts, groups like the Congressional Leadership Fund need to spend four times as many ads as a Democratic candidate in order to air the same number of ads.

One reason for the more spread out fundraising numbers for Democrats stems from the continued rise of ActBlue, a non-profit organization that helps Democrats raise money from small dollar donors across the country. The platform, which give millions of donors the option to store their information on the platform, allows each of those users to donate money to 14,000 campaigns and nonprofits with Amazon-like efficiency, dispersing money in just one-click.

This has not only led candidates to prioritize small dollar donors by focusing on creating viral moments that boost national recognition and fundraising, but it gives candidates without deep fundraising networks -- mostly those running for the first time -- the ability to raise enough money to keep up with usually well-funded incumbents.

ActBlue, in their best quarter since the organization was founded in 2004, raised a staggering $385 million in the third quarter of 2018, a haul that has been seen as a reflection of Democratic excitement. The group, according to Executive Director Erin Hill, is on track to raise over $1.5 billion this cycle.

"We have people on our side who are raising as much as the big dollars donors are, but it comes with a sense of engagement because we have millions of people behind it," said Hill, who added that the importance of small dollar donors is highlighted by the fact that it gives power to campaigns, not outside groups.

"It lets Democrats on our side spend money much smarter because campaigns can buy ad time at discounted rates and they know the messaging they are trying to get across," Hill said. "(Republicans) are spending more money and they are spending it less well. For us on the Democratic side, that is really a strategic advantage."

While Republicans have tried to create their own version of ActBlue, they have largely failed, much to the chagrin of those operatives working to protect the party's House majority who have been bewildered by the amount of money Democrats have been able to raise through the online platform this year.

The surge of Democratic fundraising has also been the result of aggressive investment by the DCCC in the early months of the 2018 midterms.

"This was an intentional strategy from day 1 to focus on candidate-side resources," DCCC national press secretary Tyler Law told CNN. "We knew the only way to compete with that amount of dark, secret Republican money is by having candidates who have the resources."

"It's why in [the PA-18 special election], people would see that [Conor] Lamb was outspent heavily by Republican outside groups, but if you actually look at amount of TV ads run, they were largely at parity," Law noted. "That was an intentional strategy -- something that the DCCC invested in."

With that strategic advantage in mind, the DCCC placed digital strategists and finance staff in regional "pods" for the first time, working with campaigns on messaging and fundraising. This created more campaigns with sophisticated fundraising operations that were able to capitalize on viral moments or breaking news that energized Democratic voters.

The committee said they also dispatched additional fundraising staff to "expansion districts," and campaigns in tougher races, in an attempt to expand the map and boost longer-shot challenger still capable of robust fundraising with the right moment.

One of the best examples of this was in the race for Texas 31, between Democrat MJ Hegar and incumbent Rep. John Carter. Hegar released a biographical web video, "Doors," in June that quickly went viral (it has been viewed nearly 3 million times on Youtube so far). In the reporting period following the video's release, Hegar outraised her opponent, incumbent Rep. Jon Carter, $1.1 million to $267,000. Carter remains the favorite to win, but Hegar will have the resources to make a contest in of a district normally out of reach.

The early focus on fundraising also allowed Democratic candidates to survive expensive, crowded primaries that took place in a series of key districts, and in particular a suite of California races, while still having the resources to continue spending in general election matchups.

"Look at the numbers -- not just people in Clinton districts, [but] in swing districts raising resources," Law said. "We have focused on races deep into the map, and the results are clear."