Tuesday's special election in Kansas to pick a successor for former Rep. Mike Pompeo, who vacated his seat to serve as CIA director under President Donald Trump, has Republicans nervous and Democrats sensing an opportunity to rally momentum for the resistance.

The heavily Republican district, which Trump won by 27 points, will decide between Republican state party treasurer Ron Estes; Democrat Jim Thompson, a civil rights attorney and army veteran; and Libertarian Chris Rockhold, an aviation expert.

But the race has turned surprisingly tight, with one internal poll circulating among Republicans showing Estes up by only one point over Thompson as of last week. In the days before the vote, both Trump and Vice President Mike Pence recorded robocalls and the National Republican Congressional Committee dumped $92,000 into ad buys for Estes—and Democrats have picked up on that. Thompson and his supporters were getting constituents fired up on social media on Monday and Tuesday under the hashtags #Resist and #FlipThe4th.

Thompson wrote on Twitter, "The momentum is on our side. The country is watching. Let's shock 'em all. Let's WIN this thing!"

"We have a chance...to make history," he said in a video message.

The momentum is on our side. The country is watching. Let's shock 'em all. Let's WIN this thing! (2/2) #RESISTANCE #Indivisible #FlipThe4th pic.twitter.com/54FptsKgpO

— James Thompson (@JamesThompsonKS) April 11, 2017

"An Estes loss—or even a win by only a slim margin—would likely be read nationally as a sign of a backlash against Trump in the heartland and be seen more locally as a renewed vitality for Kansas Democrats as they head into 2018 with the governor's office and the rest of the state's U.S. House seats up for election," Kansas City Star's Bryan Lowry reported Monday.

And Tim Murphy added at Mother Jones that Democrats should be "thrilled" about the Kansas election even if they lose. Thompson, a political novice who was inspired to run by Sen. Bernie Sanders (I-Vt.), built his campaign on small donations and no support from the Democratic National Committee.

"From last Thursday, when Republicans began to sound the alarm, through Sunday, Daily Kos raised $149,255 for Thompson through ActBlue—an astounding four-day haul for a Democrat in a district Pompeo last won by 34 points," Murphy writes. "Thompson's ability build a viable coalition in Trump country, largely on his own, may say a lot more about where the party is headed going into 2018."

Political observers have also said the outcome in Kansas could build momentum going into Georgia's special election next week.