In addition to the Kansas seat, Republicans are defending GOP-leaning seats in Georgia, Montana and SC - while Democrats are protecting a seat in a liberal California district.
In last night's special election in Kansas's 4th congressional district - a deeply red region - the Republican candidate, Ron Estes, eked out a victory over Democrat James Thompson to claim Mike Pompeo's seat, which the former representative vacated to head up the Central Intelligence Agency under President Donald Trump. Ron Estes, the Republican state treasurer, remains the favorite to win. The 17-county congressional district includes the state's largest city of Wichita, home to Koch Industries, the company led by conservative billionaire political donors Charles and David Koch.
Sanders, who came to national prominence previous year with a surprisingly strong grassroots campaign for the Democratic presidential nomination, had endorsed Thompson.
Thompson told The Huffington Post in an interview over the weekend that even if he loses, he still considers the race to be a victory for Democrats because it shows that they can make Republican strongholds competitive. So were national Republicans, who were anxious enough that the party gave Kansas more attention in the last week or two than it does in some presidential elections.
Trump tweeted about the race Tuesday morning, urging voters to get out and support Estes over his Democratic challenger.
The Democratic Congressional Campaign Committee did 25,000 live calls in the final two days before the race to support Thompson. Estes did not focus on Trump in his race, and although he is expected to be a reliable vote for Republicans in the House, he opposed the GOP health-care bill that was pulled before the congressional recess.
In the end, the race to fill now-CIA Director Mike Pompeo's seat in Congress wasn't as close as some thought, but closer than it normally would have been.More news: Vimy Ridge: Royals to commemorate defining WW1 battle
Readers of a liberal blog have donated more than $200,000 in less than a week to the Democratic candidate in a special congressional election in Kansas. Former Republican congressman Mike Pompeo won the seat by 31 percentage points in November. "A wonderful guy, I need his help on Healthcare & Tax Cuts (Reform)".
A county-by-county breakdown of results is available at the Kansas secretary of state website.
While the Kansas result suggested some blowback against Trump, another important factor was anger with the state's unpopular Republican governor Sam Brownback.
Trump says "Ron Estes is running TODAY for Congress in the Great State of Kansas". Estes performed 20 points worse than Trump did in this district just five months ago.
A loss - in one of the 100 most Republican-leaning districts in the country, according to the Cook Political Report's Partisan Voting Index scores, which measure presidential-election performance against the national average - would have been devastating for the GOP and particularly for Trump, who already in his young presidency has had trouble wrangling lawmakers to vote for administration priorities like the repeal of Obamacare.
Now, the narrow win for Estes has some Republicans concerned. Ted Cruz of Texas campaigned for him.
So how desperate must Democrats and their pals in the press really be if they're doing cartwheels over losing by twice as much in a district which, having been represented by a Democrat for 18 years until 1995, is nowhere near as red as OH-02?