Let’s start with the four “by the numbers” things you need to know about the important election happening Tuesday night in Pennsylvania’s 18 th district.

1. It’s a district that the Cook Political Report scores at R+11. That’s normally a safe Republican seat. It’s in the range of some other GOP districts that have held special elections since Donald Trump became president that were a lot closer than expected. That Georgia district that Democrat Jon Ossoff almost won was R+8. The Kansas district that started as a blowout and ended as a narrow win for the Republican was R+15. And the Montana district won last year by the Republican journalist-puncher was R+11.

They were all close—but the Republican won every time. (You can see all these district ratings here if you’re interested.)

2. The district backed Trump over Hillary Clinton 58-39. And according to FiveThirtyEight’s weighted average of presidential results, the district is 21 points more Republican than the country as whole.

3. Perhaps offsetting 1 and 2 is the fact that nearly half of the vote in this district comes from the suburbs south of Pittsburgh (here’s a good map you can zoom in tight on). Allegheny County, where these suburbs are, accounted for 43 percent of the district’s vote in 2016, and while it went narrowly for Trump, it’s winnable for the right kind of Democrat. If Democrat Conor Lamb is going to have a chance to pull it out Tuesday, he’ll need to do a lot better than Clinton did in these south Pittsburgh suburbs. If Steve Kornacki is madly circling these areas Tuesday night as they populate blue, Lamb will win.

4. The money: While Lamb has raised more than Republican Rick Saccone, outside GOP groups are spending on this race by a ratio of about six- or seven-to-one over Democratic groups. A whopping $10.5 million in outside money is going to support Saccone or, mostly ($7.5 million), to attack Lamb. And these outside groups are up to their usual tricks. Politico reported Monday that one outside conservative group sent mailers to identified liberal households in the district thanking Lamb for “opposing gun restrictions” to depress liberal turnout.

I know, or once knew, this district pretty well. Morgantown, my hometown, is just across the state line. I spent a lot of time driving back and forth between Morgantown and Pittsburgh and exploring these towns in between, and going to South Hills Village in Mount Lebanon on shopping excursions (what a treat that was, before we had malls in Morgantown!). It’s the kind of place that was completely Democratic when I was a kid but these days, because of guns and the culture wars and now Trump, is totally Republican—except for one thing.

It’s still union. Not the way it was 40 years ago, but more than most places. Of course “union” doesn’t mean “Democratic” the way it used to, since white male union members (most of the union members in this district) vote Republican. But the unions are in this race for Lamb in a big way, and there are indications that the on-the-ground enthusiasm for Lamb is strong.

The result here may well pivot on whether the unions can get enough of their voters to vote union over Trump Everybody makes a big deal about this being primo Trump Country, and it is. The tariffs are wildly popular there. On the other hand, it’s hardly clear that times are any better there since Trump took office. A mine in southwestern Pennsylvania closed down just last week. It’s just outside the district but close enough that many of the miners who worked there live in the 18 th , and the closing will eliminate 370 jobs in total.

The closing doesn’t have anything to do with Clinton or Barack Obama or the EPA. Coal is losing to natural gas. Five new natural gas power plants are scheduled to open in southwestern Pennsylvania in the next few years. This is where things are headed, no matter who’s president. And ironically, Saccone isn’t a Trump Republican, despite his current claims to the contrary. He’s a Tea Party guy. He’s voted for right-to-work laws.

If Saccone holds on and wins, Republicans will exhale. They’ll decide that they won it in no small part by making Nancy Pelosi the issue, and they’ll keep doing that in district after district to the tune of hundreds of millions of dollars.

And if Lamb wins? Look out. Republicans will be total panic mode.

There are 45 House seats that have Cook ratings of R+11, R+10, or R+9—in other words, that are basically similar to this seat ideologically. That’s 45 Republicans (or somewhat fewer; some are deeply entrenched incumbents) who’ll wake up terror-stricken on Wednesday if their party has lost a district that Trump won by 20 points. You’ll see a lot more Republican retirements in the next few weeks. The wave will be on its way to shore.