UPDATE (April 12, 7 a.m.): Republican Ron Estes won the U.S. House special election in Kansas’s 4th congressional district on Tuesday. But Estes beat Democrat James Thompson by just 7 percentage points. In the original article below, we had said the Kansas 4 result should worry the national GOP if Estes wins by any margin less than 20 points.

No Democrat has won a House race in Kansas’s 4th Congressional District in over 20 years. It’s a Republican district, full stop. So, even a not-so-bad loss by Democrat James Thompson in Tuesday’s special election against Republican Ron Estes would be a worrying sign for the GOP.

In fact, Republicans seem a bit anxious about the race. The National Republican Congressional Committee rushed in at the last minute to spend some money in the district. And Ted Cruz came in late to campaign for Estes. Thompson is still the underdog, but anything less than a blowout victory by Estes would be a flashing warning siren for Republicans ahead of next week’s more high-profile race in Georgia’s 6th Congressional District, and a preliminary sign of weakness for the GOP in the lead-up to the 2018 midterms.

To be clear, no one really knows exactly what to expect in Kansas 4. There have been no public polls. A survey conducted for Thompson’s campaign in March put Estes ahead by 24 percentage points. But thanks to an unpopular Republican president in Donald Trump and an unpopular Republican governor in Sam Brownback, the race has seemingly tightened since then. An internal Republican poll taken last week supposedly had Estes up only 1 point. Both the Cook Political Report and Inside Politics, while showing Estes favored, have downgraded his advantage recently. Republican turnout also looks relatively weak in early voting (which, of course, may not end up meaning anything.) And in a special election where turnout is expected to be low, small differences in who votes can have big consequences.

No Democrat occupies a seat as Republican leaning as Kansas 4 has been over the last two presidential elections. Kansas 4 is 29 percentage points more Republican-leaning than the nation as a whole, according to a weighted average of the last two presidential election results. In fact, only 19 percent of all congressional districts have been as Republican-leaning in the last two presidential elections. For comparison, a GOP loss in Georgia’s 6th District would scare Republican political operatives, and it’s just 9 points redder than the nation.

Nevertheless, Kansas 4 could be just as instructive for understanding the national environment as Georgia 6. We know from past special elections that wins and losses matter less than the margins if what you’re interested in is the upcoming midterm election. Democrat Paul Hackett, for example, did much better than expected in Ohio’s 2nd District in 2005, but lost to Republican Jean Schmidt. The closeness of that race, however, foreshadowed big gains for the Democrats in the 2006 midterm election.

A Thompson loss of 20 percentage points or less would probably be a good sign for Democrats. That’s 9 points closer than the past presidential vote suggests the margin should be in a neutral political environment. If Democrats win the national House vote by 9 points in 2018, that’d probably be good enough to take back the House. Of course, Kansas 4 is just one election. Democrats will need to overperform similarly in multiple races before it’s safe to conclude anything meaningful about 2018. They already did so last week in California 34’s special election. If they continue to do so, it’s a sign Trump’s unpopularity is hurting Republicans down ticket. If, however, Estes and other Republicans are not underperforming the past presidential vote, it could be an ominous sign for Democrats in 2018.