The Republican Party is pouring significant money into a Kansas special election for Congress in the final days of the campaign ― an early sign that voter backlash to President Donald Trump is creating political dividends for Democrats in one of America’s most conservative pockets.

The Republican spending in the 4th Congressional District race, first reported by the Wichita Eagle, is likely a response to a surge in Democratic enthusiasm in the district, which happens to be the base for Koch Industries, the conglomerate owned by the right-wing megadonor Koch brothers.

Early voting showing higher-than-normal Democratic turnout in the predominantly Republican district suggests that Republicans have reason to be worried.

Although Republicans will maintain control of both chambers of Congress until 2018, at least, a strong Democratic showing in Kansas and four other U.S. House special elections before June could still affect Trump’s agenda in the next two years.

“Republicans are not going to stand up to this guy if they see that their re-election is in jeopardy when they stand with him,” said Tom Bonier, CEO of Democratic electoral data strategy firm Target Smart. “This is an indicator of what’s to come and whether or not Republicans are going to be rubber-stamping Trump or standing up to him.”

The Kansas seat opened when Trump tapped tea party stalwart Mike Pompeo to head the Central Intelligence Agency. The district, which covers a south-central swath of the state that includes Wichita, re-elected Pompeo by more than 31 percentage points in November.

But now, as the Wichita Eagle reports, the National Republican Congressional Committee is injecting $92,000 into the deep-red district in the campaign’s final days. The Washington-based campaign arm for House Republicans bought $67,111 worth of television ads, according to a local news channel’s disclosure form obtained by the Eagle.

The NRCC is spending an additional $25,000 on digital advertising, according to a Federal Election Commission disclosure. Half of those funds are earmarked to boost Ron Estes, the Republican candidate, and the other half will target Democrat James Thompson.

Mark Reinstein/Getty Images The National Republican Congressional Committee is spending money to defeat James Thompson, the Democratic candidate in Kansas' solidly red 4th congressional district.

Thompson, a civil rights attorney and Army veteran, is trying to capitalize on public opposition to the hard-line conservative agenda of Kansas Gov. Sam Brownback (R). Brownback on Friday vetoed an expansion of Medicaid passed by the state’s Republican-controlled legislature.

Estes, the Kansas state treasurer, emphasizes budget savings and his fiscally conservative record.

Absentee voting by mail began across the 4th District on March 20. Early in-person voting also began at the end of March. Election Day is Tuesday.

In 10 out of the 17 counties where early in-person voting data were available, 41 percent of voters were registered Democrats and nearly 48 percent were registered Republicans as of Wednesday, according to Bonier’s analysis of state figures. (Although the Democratic National Committee is one of Target Smart’s clients, the firm is not working on the special election in Kansas in any capacity, Bonier said.)

@Nate_Cohn Here - this is just the early vote for the KSCD04 by county and party. I'll share VBM separately: pic.twitter.com/PHCRRrWrI9 — Tom Bonier (@tbonier) April 5, 2017

In the district’s 2016 general election, by contrast, registered Democrats made up 23 percent of the electorate, while nearly 52 percent of voters were Republican.

Tom Bonier Registered Republicans dramatically outnumbered registered Democrats in the 2016 general election.

Democrats have narrowed the gap in absentee voting by mail as well. Thus far, 43 percent of absentee voters across the district are registered Democrats, while 45 percent are Republicans.

That’s a major change from 2016, when Republicans topped Democrats in absentee voting by 29 percentage points.

@Nate_Cohn Here's the final '16 electorate for KS04. Dems were -29%, as compared to only -2% in the '17 VBM thus far. pic.twitter.com/zvuVASbpIK — Tom Bonier (@tbonier) April 5, 2017

Bonier also found evidence that registered Democrats who did not vote in the 2014 midterm elections ― people sometimes known as casual voters ― are turning up this time. Of the 1,326 people who did not participate in the 2014 midterm elections, but have already voted in the special election ― either in person or by mail absentee ballot ― 44 percent are registered Democrats and 29 percent are registered Republicans.

Interesting: In the KS CD4 special election, 1,326 of the people who have already voted didn't vote in '14. These people are 44% D, 29% R. — Tom Bonier (@tbonier) April 6, 2017

Early voting patterns, of course, are a notoriously inconsistent indicator of final election outcomes. Registered Democrats do not necessarily vote for the Democratic candidate, and turnout on Election Day itself is often what makes the difference.

Early voting in North Carolina and Florida last fall, for example, appeared to favor Hillary Clinton. She ended up losing both states.

Bonier acknowledged these caveats. But Democrats’ near parity with Republicans in absentee voting, where the GOP often does better, suggests this year could be different.

And, in such a heavily Republican district, Democrats do not need to win to demonstrate that the wave of anti-Trump enthusiasm has electoral staying power. Holding a Republican margin of victory to 15 percentage points would be a huge accomplishment, Bonier said.

The special election in Kansas has largely flown under national Democrats’ radar. The Democratic Congressional Campaign Committee has not invested in the race, but the organization seemed pleased that Republicans felt compelled to do so.

“President Trump won this traditionally red district by 27 points just a few short months ago, so the fact that the NRCC is panicking now confirms how strong the headwinds are that Republicans face across the country,” DCCC spokesman Tyler Law said in a statement.

The national Democratic Party and liberal activists have focused heavily on the contest for Georgia’s 6th Congressional District, where 30-year-old filmmaker Jon Ossoff is running a competitive race in the seat vacated by Health and Human Services Secretary Tom Price. High Democratic turnout in early voting in Ossoff’s district bodes well for him, too.

Like the Kansas race, the battle to fill Montana’s House seat has gotten less national attention.

But the Democratic candidate in the district, Rob Quist, a popular banjo player who supported Bernie Sanders’ presidential bid, has generated substantial enthusiasm across the state, raising the possibility of an upset there as well.