Most analysts agree that this is likely to be a bad year for the congressional GOP. There is disagreement as to just how bad it will be. This late in the season, we can look for crystallizing moments that summarize the challenges facing the incumbent party. One such crystallizing moment can probably be found in Tuesday’s special election for Ohio’s 12th Congressional District.

On its face, the 12th is a poor choice for a bellwether district; the heavily Republican 12th and its predecessors have sent Republicans to Congress in every election dating back to 1980 (and 1936 before that). But the weakness of the modern Republican Party among college-educated whites will hit hard here, and we could see an upset that presages a wipeout in November.

On its face, the 12th is a varied district. One arm extends to the east to take in Zanesville, on the outskirts of the Allegheny plateau. A second arm extends northward to encompass Mansfield, an old manufacturing center that is home to the famous prison used as a set-piece for “The Shawshank Redemption.” These areas of the district were strong for President Trump: He won about 70 percent of the vote here.

But although these “arms” take in a large portion of the land area of the district, they cast only about a third of the vote. The 12th is largely a suburban district and is one of the best educated in the state. The remainder of the district’s population is split roughly evenly between Licking, Delaware, and Franklin counties, with a plurality actually living in Franklin.

Licking County is anchored by the old industrial center of Newark and contains a heavily Democratic pocket in Granville (Denison University). The growth, however, is occurring on the western edge of the district, where places like New Albany are enjoying substantial exurban development. The county actually swung more toward Republicans in the 2016 elections: While Mitt Romney won 56 percent of the vote, Donald Trump improved to 61 percent. The extent to which these blue-collar voters turn out and pull the lever for Republicans this cycle may hold the key to Republicans’ keeping control of the House.

Delaware County, to the north of Columbus, is a different story. While there are rural areas, and an old industrial town (Delaware, which is also home to a small college), the bulk of the vote will be cast in the wealthy suburban/exurban portion of the district. These voters are traditionally Republican, and Delaware County hasn’t voted for a Democrat since Woodrow Wilson beat Charles Evans Hughes here in a landslide in 1916 (it was also one of five Ohio counties to back Barry Goldwater in 1964).

But lately the trend is away from Republicans. George W. Bush carried the county by 36 percentage points in 2000 and 33 in 2004. McCain won by 20; Romney improved slightly to a 22-point win here, though that was less than his nationwide improvement. Donald Trump, however, won here by just 17 points.

Finally, there is the Franklin County portion of the district. The area includes Dublin, which is an upscale, wealthy suburb of Columbus, as well as Westerville, which is more solidly middle class. The district also juts into the heart of Franklin County, dipping down into the inner suburbs of Upper Arlington and Clintonville, and taking in a student section east of High Street (if you live in a fraternity or sorority house, you go to school in the 3rd District but vote in the 12th).

All of this adds up to a district that is clearly Republican – Trump and Romney both carried it by about 10 points – but where Republicans have potential vulnerability in a wave election.

That appears to be what is happening. Pat Tiberi, who was elected to replace John Kasich in 2000, opted to resign from Congress this year, setting up Tuesday’s special election. Unlike some other special elections, where unexpectedly close outcomes could be attributed to local issues (KS-4) or weak candidates (PA-18), both parties nominated relatively typical candidates. Republicans opted for state Sen. Troy Balderson, while Democrats went with Franklin County Recorder Danny O’Connor.

The polling initially showed Balderson (pictured) with a substantial lead, with two June surveys showing him ahead by an average of 10 points. But Balderson was below 50 percent, and the undecideds seem to have broken hard for O’Connor (Balderson has been virtually absent from the airwaves, suggesting fundraising difficulties). O’Connor has released polling showing him closing the gap, and a recent Monmouth poll has Balderson up by one.

If Balderson loses it will be difficult to spin this as anything other than bad news for the GOP. Yes, most races will feature incumbents, but there are a raft of open seats in similarly suburban areas that are more Democratic than the 12th. Quite frankly, even a close race would likely signal substantial GOP losses.

The usual caveats apply here. This is a special election, though again, it lacks any quirky features that often set special elections apart. More seriously, there is a core base of Democrats who are amped up and will crawl over broken glass twice to vote against Republicans. Their numbers will probably be diluted in the general election. But make no mistake about it, if the election resembles the polling we’re seeing right now, it is a terrible sign for House (and Senate) Republicans.