Will Donald Trump’s last-minute intervention make the difference in a key special House election? Voters go to the primary polls in five states today, but the main event will take place in Ohio’s 12th Congressional district. It’s been in Republican hands for 35 years, but Patrick Tiberi’s early retirement has left the door open to Democrat Denny O’Connor, setting up a contest that will test Trump’s ability to turn out voters to support his agenda in November:

The most closely-watched election of the day will take place in suburban Columbus, Ohio, where Democrat Danny O’Connor is in a close contest against Republican Troy Balderson in a preview of the November midterms in which control of the House could be decided in similar communities nationwide that have long voted for Republicans but are souring on Trump.

Although the GOP has held the district since 1983, the party and conservative PACs have spent more than $4 million to stop a Democratic surge, compared to less than $1 million from Democratic groups. On Saturday, the president stumped with Balderson, and the candidate said he would win if Trump supporters put in the work.

“Your countless volunteer hours, your enthusiasm and, most importantly, your vote helped put Donald J. Trump in the White House,” Balderson said at the rally. “We need to do that again.”

The winner of Tuesday’s race will serve out the term of retired Republican Patrick J. Tiberi, and both O’Connor and Balderson will face off again in November. Both parties agree that Democrats won the early vote, returning more absentee ballots than Republicans in a district where the president’s party has usually begun with a lead. To win, Republicans say they’ll need to overcome O’Connor’s lead in Franklin County, the biggest population center, with strong rural and suburban turnout.

Trump traveled to the district on Saturday, holding a rally for Balderson and warning attendees about the consequences of failure. A win for O’Connor, Trump later amplified on Twitter, would install “a total puppet for Nancy Pelosi and Maxine Waters.” The last-minute personal intervention is rare for a special election, but apparently was necessary. Balderson had led two months early by a comfortable (if not exactly decisive) 43/33 margin in a Monmouth poll. By the time Trump arrived on Saturday, that had changed to a 47/46 lead for O’Connor in an Emerson poll, with early voting limiting Trump’s potential impact.

That’s a warning signal from a district with an R+7 Cook index rating. With the exception of a lone Democratic win in 1980, this district has been in Republican hands since the 1938 election, an 80-year run. Tiberi held it for nine terms, and a fellow named John Kasich held it for the nine terms prior to Tiberi.

That might be part of Balderson’s problem in today’s race. Balderson wasn’t Trump’s original endorsement; he favored Melanie Leneghan, who ran on more of a MAGA platform in the May primary. Leneghan barely lost that race, but Balderson has tried to broaden his populist appeal since then. Kasich’s opposition to Trump might still be hanging over the district and perhaps Balderson himself, as might Trump’s waning popularity in Ohio — although one might think that the R+7 district would have enough of a buffer to deal with it.

Republicans predict a win tonight, but have started downplaying the result too:

The Republican counter-argument, reduced to its essence, is: Don’t get carried away.

Most GOP insiders expect Balderson to eke out a victory on Tuesday — “I still think it is Troy Balderson’s to lose,” said Gonidakis — and they also caution that the idiosyncrasies of special elections need to be borne in mind.

This is particularly important, they say, when Election Day falls in early August, when many Ohioans are on vacation or thinking about the imminent start to the school year.

“We shouldn’t read too much into this particular election,” Weaver insisted. “The district will return to its normal voting patterns in November.”

Normally that would be sensible advice, but there are two factors which might argue against it. The first is the dyed-in-the-wool-Republican nature of the district, which shouldn’t change that much even in a special election. The second is that this special election is close enough to be part of the engagement over the regular election cycle. It’s not a special election in January, way off the beaten path; the midterms are 90 days out. If Balderson loses, it’ll be tough to explain away as an anomaly rather than a harbinger of a very tough midterm cycle.

That’s still a big if in a R+7 district, although Trump is taking no chances … and neither is Balderson: