In the dead of the summer, a wave test looms.

The venue is Ohio's 12th Congressional District, where a special election to fill the seat vacated by Republican Rep. Pat Tiberi is scheduled for Aug. 7. Taking in all or part of seven counties around Columbus, the district backed Donald Trump by 11 points in 2016 and Mitt Romney by 10 before that.

Normally, it would be a lock for the GOP.

But Democrats are hoping 2018 will prove to be a wave year, and there's already plenty of evidence to support their optimism. A tight contest here — or an outright Democratic victory — would significantly reinforce the idea that the party is in for big gains in November. A solid Republican win, on the other hand, would provide a fresh dose of optimism for the GOP that its 23-seat House majority can be saved.

On paper, there's reason to suspect Democrats can make it a game.

Just consider the House special elections already held since Trump took office. So far, there have been seven that pitted a Democrat against a Republican (a June 2017 special election in California's heavily Democratic 34th Congressional District did not include a Republican in the runoff), and five of them have featured sizable movement toward the Democrats relative to the Trump-Hillary Clinton result.

On average in these races, Democrats have slashed the Trump-Clinton margin by 10.5 points — almost exactly the spread by which Trump carried Ohio's 12th District in 2016. So it's not hard to imagine the Aug. 7 contest becoming a barnburner.

Hovering over this race, of course, is the memory of the March special election in Pennsylvania's 18th District, where Democrat Conor Lamb defeated Republican Rick Saccone. For Republicans, losing a seat in a district Trump had carried by 20 points was traumatizing, raising the specter of wide-scale electoral carnage this fall.

There are some obvious parallels between that contest and this one, including the fact that both districts sided strongly with Trump in 2016. There is also a clear generational contrast with each. In Pennsylvania, Lamb, at 33, was more than 20 years younger than Saccone, a veteran state legislator. In this race, Democrats have nominated Danny O'Connor, a county official in his early 30s, while Republicans are backing Troy Balderson, a 56-year-old state senator who's been in the legislature for nearly a decade.

Danny O' Connor, a Democrat, is running for Congress in Ohio against GOP state Sen. Troy Balderson Danny O'Connor For Congress via Facebook

That said, the race is not yet a repeat of the Pennsylvania special. A Monmouth University poll released last week put Balderson ahead, 43 percent to 33 percent. More encouragingly for the GOP, the poll also tested what it called a Democratic "surge" scenario — disproportionately high turnout in the district's Democratic areas — and still found Balderson up by seven points.

Nationally, Republicans are hopeful that there have been subtle but important shifts in the political climate since the Lamb-Saccone race.

The president's approval rating averages about 43 percent right now, higher than it was for much of 2017 and early 2018, and he appears to have consolidated support within his own party to an unusual degree. In May, Republicans also closed to within four points in the generic congressional ballot test — a level at which they believe they can maintain their majority — though in the past two weeks that number has spiked back to seven points.

Troy Balderson, a Republican state senator in Ohio, in 2012. He is running for Congress against Danny O'Connor, a Democratic county official. James Miller / The Marion Star via AP file

Additionally, the political character of Ohio's 12th District differs significantly from Pennsylvania's 18th, an area that up until around the turn of the century was a Democratic bastion and where registered Democrats still outnumber Republicans. It may be that there was more readiness among the locals there to cast a Democratic ballot than there is in Ohio's 12th, where there's a more embedded GOP tradition.

For now, Democrats can find solace in the fact that the initial polling in Pennsylvania — also conducted about two months before the special election — was similarly lopsided in the GOP's favor. Monmouth's survey shows that O'Connor isn't well known and suggests that voters are not engaged with the race. This can, of course, change, as it did in Pennsylvania, and O'Connor is slated to launch his first television ads in the coming week.

Nor is the district's deep Republican pedigree necessarily a dealbreaker for Democrats. Arizona's 8th District, where the most recent House special election was held in April, had no particular history of Democratic allegiance, and yet Democrats still managed to shave 15 points off of Trump's 2016 margin.

This race figures to attract more national attention as Aug. 7 approaches. Just on Friday, Vice President Mike Pence headlined a fundraiser for Balderson. Democrats don't need to win this race to win back the House, but given the pattern of all the other special elections, they would be disappointed if they can't at least make the Republicans sweat.