Senate Majority PAC spent $3.55 million on the effort, and Women Vote! added $1.75 million, officials in those organizations told POLITICO. Those details will be revealed publicly Thursday because those groups and Sunflower State face a monthly filing deadline with the Federal Election Commission.

The effort failed to elevate Kobach to victory, which would have made the race a top-tier battleground and boxed in Republicans given their consistent opposition to him. But Democrats involved said the meddling was successful because they worked to degrade Marshall’s image with attacks that would translate to a broader electorate, essentially jump-starting the ad war of the general election.

“It was a buffet of riches. You have extraordinarily weak candidates running against each other,” J.B. Poersch, the president of Senate Majority PAC, told POLITICO in describing the effort. “You had to open the possibility that any of the candidates, either Marshall or Kobach, could come out of that, and you wanted a weakened scenario.”

Following the primary, both parties see Kansas on the fringes of the Senate battlefield. Though Democrats have not won a Senate race there since the Great Depression, Republicans aren't taking it for granted as the party works to defend its endangered majority.

This is not the first time national operatives have established groups that meddled in opposing party primaries without revealing their funding sources until after the elections. A similar scenario played out in 2018, when Republicans stood up a group to stop a controversial candidate in West Virginia, and Democrats funded a second group to bash the other Republicans in the race. Earlier this year, Republicans funded a group in North Carolina Senate primary that unsuccessfully boosted a challenger to Democrats’ preferred nominee.

Poersch said that his group got involved after another pop-up super PAC began spending in the race. Plains PAC was run by a Republican operative and spent millions attacking Kobach as an unacceptable nominee. That group will also reveal its funding source this week. Plains PAC and Marshall's campaign both highlighted the Democratic meddling in campaign ads during the primary.

Poersch defended the effort as successful for forcing Republicans to spend more heavily in the primary even as they face expensive contests in more competitive states. Plains PAC spent $3.3 million, and Senate Leadership Fund, a group run by allies of Majority Leader Mitch McConnell, spent nearly $2 million in the primary. Sunflower State ultimately spent $5.3 million.

According to still-unofficial results, Marshall defeated Kobach, 40 percent to 26 percent.

“They were signaling not only they were nervous about it, but that it was shaky,” Poersch said. “They won by a decent margin, but at the point all indications were that it was shaky.”

EMILY’s List, which endorsed Bollier, also funded the effort with $1.75 million through Women Vote!, the group's independent-expenditure arm. Ben Ray, a spokesperson for EMILY’s List, said the effort gives Democrats a better chance of flipping the seat in November, pointing to $4 million a nonprofit aligned with McConnell announced it would spend following Marshall’s primary win.

“Congressman Marshall's unpopular record and the strength of Barbara Bollier's focus on results over politics are why this is a margin-of-error race today. And Republicans know it, because they're spending more than $4 million of Mitch McConnell's money to try and prop Marshall up,” Ray said in a statement.

Republicans supported Marshall after Secretary of State Mike Pompeo declined overtures to run. GOP officials at the time of Marshall’s primary victory saw it not only as a defeat of Kobach, who they had repeatedly warned would put the Senate majority in danger, but as a victory over Democrats. Steven Law, a former McConnell chief of staff and president of Senate Leadership Fund, said on the eve of the Aug. 4 primary that it was the “first skirmish to stop Chuck Schumer’s brazen Senate power grab,” and that Republicans “saw right through” the efforts to elevate Kobach.

Poersch downplayed the focus on Kobach, pointing out most of the ads were directed at Marshall. But he also brushed aside any concerns from liberals about elevating Kobach, a hard-line conservative who has promoted crackdowns on immigration and tightening voting access laws, should he have won the primary.

“Kobach was a dirtbag and a loser, and it was only a matter of time before he lost,” Poersch said. “Whether it was the primary or general, he was going to lose at some point.”