In October and September alone, the PAC and its affiliated nonprofit, American Action Network, spent $1.65 million on digital independent expenditures to supplement its television advertising efforts.

That’s more than three times what House Majority PAC, the Democratic counterpart to CLF, spent on digital efforts for the entire cycle.

The expenditures buy the groups pre-roll ad space before YouTube videos and on other video streaming sites, as well as banner ads on websites and search words. CLF says their digital focus makes particular sense in the waning days of the election, as voters are likely to turn to the internet to learn more about candidates.

“In the final week of the campaign, while voters are tuned in and trying to make their decision about candidates, they are getting delivered our ads constantly online and we're able to own search in the final weeks,” said CLF spokesman Dan Conston.

The group purchased half the ad space on the Des Moines Register’s website, in hopes of swaying voters turning to their local news source for information on the Iowa 3rd district fight between Rep. Leonard Boswell (D) and Tom Latham (R), one of the most closely-watched races in the nation.

And in both Texas’s 23rd district, where Republican Quico Canseco is facing a challenge from Democrat Pete GallegoPete Pena GallegoTexas Democrats smell blood in the water for 2018 ObamaCare repeal vote: 15 GOP lawmakers to watch Vulnerable Texas GOP lawmaker survives rematch MORE, and California’s 24th district, where Abel Maldonado is challenging Democratic Rep. Lois Capps, the groups bought out nearly all search terms related to both candidates.

“You're capturing somebody that actively wants to engage with the campaign,” Conston said. “In many places it does continue to reach people that maybe have tuned out of TV.”

While television ads may be seen by thousands more people than an online ad, online ads can be targeted to hit the voters that matter: Undecideds and swing voters.

Using already-available data, CLF and AAN can focus in on specific subsets of voters in a given district and send them ads across a number of websites they might visit while browsing the internet.

Chris Georgia, CLF’s digital director, said the groups’ digital efforts can play a role in getting out the vote as well.

“For active GOP voters, in the last weeks of the campaign we will be targeting them in higher frequency to get them to the polls,” he said.

While the groups have launched digital ads in nearly every one of its races, online advertising makes particular sense in expensive television markets, where a dollar can go much farther online than it can on the airwaves.

In Illinois’s 10th district, which is served partially by the expensive Chicago media market, the group is spending more online there to compensate for the fact that they can’t buy as much television time as they might in a less expensive market.

“The cost is consistent around the country. Whether we're going in to Chicago or Duluth, it costs us the same amount of money to hit that person with an ad online,” Georgia said.

Digital is by no means the new frontier it was in 2008, when President Obama’s campaign revolutionized the use of the internet and social media, and CLF and AAN have engaged in digital campaigns since then. But the effort is increasing, Conston said, as digital becomes “the key supplement” to the group’s television campaigns.

“It’s an additional voter contact. The goal is to hit our target voter as many times as possible with the same message, across multiple platforms, and digital is absolutely important and effective at doing that,” he said.

--This post was updated at 1:09 p.m. to reflect that CLF is spending in California's 24th district.