March came in like a lion; will it go out with a Lamb? Voters in Pennsylvania’s 18th Congressional District will decide that on Tuesday as they vote in the first of many federal elections in 2018. Like in previous races for Georgia’s 6th Congressional District and Alabama’s U.S. Senate seat, the party that wins the Pennsylvania special election will be said to have the momentum heading into the pivotal midterm elections later this year. In our view, that’s a bit overblown — in reality, it’s the margin of victory that tells us what to expect in midterm results, and Democrat Conor Lamb is likely to overperform in Pennsylvania no matter who wins. We will still be watching that margin closely, though, and a decisive victory by Republican Rick Saccone could be a sign that that something in the political environment has changed. So here’s what you need to know to follow the race.

1. The partisanship

This seat became vacant in October 2017 after the resignation of GOP Rep. Tim Murphy, who admitted to an extramarital affair and allegedly asked his mistress to have an abortion despite his stated pro-life views. Before the scandal, the Republican had won eight consecutive elections in this district by margins of no fewer than 15 percentage points. In both 2014 and 2016, no Democrat even bothered to run against Murphy. President Trump carried the 18th District by nearly 20 points (58 percent to 39 percent); Mitt Romney won it by a similar margin in 2012. According to FiveThirtyEight’s weighted average of presidential results, the Pennsylvania 18th is 21 percentage points more Republican-leaning than the nation as a whole.

That’s very red — but so were most of the other places where we’ve seen special elections so far this cycle, and Democrats in the vast majority of those races have consistently done better than the partisan leans of their districts would predict. The margin in federal special elections specifically has moved 16 points toward Democrats compared to each constituency’s usual partisan lean. If that happens again in Pennsylvania, Democrats will be nipping at Republicans’ heels.

How will Dems perform in Pennsylvania’s 18th district? Special elections this cycle, by the constituency’s partisan lean* and final vote margin Year Date Seat Partisan Lean Vote margin Dem. Swing 2017 April 4 California 34th† D+69 D+87 +18 April 11 Kansas 4th R+29 R+6 +23 May 25 Montana At-Large R+21 R+6 +16 June 20 Georgia 6th R+9 R+4 +6 June 20 South Carolina 5th R+19 R+3 +16 Nov. 7 Utah 3rd R+35 R+32 +3 Dec. 12 Alabama U.S. Senate R+29 D+2 +31 2018 March 13 Pennsylvania 18th R+21 ? ? * The average difference between how the constituency voted and how the country voted overall in the last two presidential elections, with 2016 weighted 75 percent and 2012 weighted 25 percent. † Primary results used because both general-election candidates were from the same party. Primary included multiple Democratic candidates, and results show vote share for all Democratic candidates combined. Source: Daily Kos Elections, secretaries of state

But there’s reason to believe the swing in Pennsylvania could be even greater. First, in special elections for Pennsylvania state legislative offices this cycle, Democrats have outperformed expectations to an even greater degree: a 29-point shift in the margin. Second, the Pennsylvania 18th is actually ancestrally Democratic. Before Murphy was elected in 2002, the area was represented by Democrat Frank Mascara, who opposed abortion and gun control and projected a blue-collar image. That legacy lingers today in the fact that there are still more registered Democrats in the district than registered Republicans (46 percent of registered voters to 41 percent). Even though they’ve grown accustomed to voting Republican for president, voters in the 18th clearly aren’t opposed to casting a ballot for Democrats under the right circumstances. For example, the area currently within the 18th Congressional District voted for Sen. Bob Casey, who also campaigned as a pro-life, pro-gun Democrat, by 11 points in 2006.

2. The players

And lo and behold, Lamb fits the same socially-conservative-Democrat mold. A 33-year-old former federal prosecutor and Marine, Lamb says he is personally opposed to abortion and opposes new gun control laws. His GOP opponent Saccone, meanwhile, is a four-term state representative who brags he “was Trump before Trump was Trump.” Saccone is known as a rock-ribbed conservative but not as a strong fundraiser: He has raised almost $920,000 for the race and claimed just over $300,000 cash on hand as of Feb. 21. By contrast, Lamb has raked in $3.9 million and was sitting on a nearly $840,000 war chest.

Saccone won’t lack for resources, however, thanks to the GOP cavalry. As of Friday, independent-expenditure groups, including the Congressional Leadership Fund and National Republican Congressional Committee, had spent almost $10.5 million to prop up Saccone; Democratic outside groups had put in just $1.8 million. Even with Lamb’s $3.1 million in spending, liberal forces are being heavily outspent. The Democratic dollars are going farther, however, because candidates benefit from lower TV advertising rates than outside groups do. As of the beginning of March, 42 percent of broadcast TV ads aired so far were pro-Lamb, while 58 percent were pro-Saccone.

3. The polling

Four pollsters have surveyed the Pennsylvania 18th in the past month, and they all agree that the race is within the margin of error. Here are the latest numbers:

Latest polls of the Pennsylvania 18th special election As of March 12, 2:45 p.m. Dates Pollster 538 Pollster Rating Lamb Saccone Mar. 8–11 Monmouth University A+ 51% 45% Mar. 6–8 RABA Research C+ 48 44 Mar. 1–5 Gravis Marketing B- 42 45 Mar. 1–3 Emerson College B 48 45 Average 47 45

The two candidates are almost tied in the average of the polls — Lamb has a 2-point lead (Monmouth, the only FiveThirtyEight gold-standard pollster to look at the race, has Lamb ahead by 6 points ). However, despite the fact that the polls show a tight race, even a runaway win by one of these candidates wouldn’t be that much of a shocker. Polls of U.S. House races already carry a lot of uncertainty, and they’re even less reliable for special elections. Since 2004, polls taken within three weeks of a special House election have missed the final result by an average of 5 percentage points, and their true margin of error was 13 percentage points in either direction.

4. The political geography

Taking in coal mines, covered bridges, wealthy suburbs and lush state parks, the Pennsylvania 18th stretches from Pennsylvania’s southwest corner up north past Pittsburgh International Airport to the Ohio River, then cuts east across the Monongahela River to the Laurel Highlands. It encompasses parts of four counties: Allegheny (home to 43 percent of the district’s voters, based on 2016 presidential results), Westmoreland (33 percent), Washington (22 percent) and Greene (2 percent).

The 18th District’s slice of Allegheny County consists of Pittsburgh’s southwestern suburbs and is a swing region — Democrats typically find most of their votes here. The other three counties, more rural and far-flung, are normally Republican strongholds, although they have strong historical ties to organized labor, a traditionally Democratic constituency.

If you’re watching the special election results as they come in (polls close at 8 p.m. Eastern), pay attention to these county-by-county variations; they can help you project the ultimate winner. Assuming that the whole district shifts left or right uniformly (more on this in a second), we’ve calculated an approximate benchmark for how we would expect each county to vote if the election were exactly tied districtwide. If the counties are voting more Republican than their benchmarks, then Saccone is on pace to win. If they’re voting more Democratic, then Lamb is ahead. (The not-so-grisly math behind these benchmarks: We used the same weighted average of the last two presidential elections to arrive at a partisan lean for each county, then moved it 21 points in Democrats’ direction — because that’s how much the entire district needs to shift leftward to deliver Lamb a victory.)

County benchmarks for Pennsylvania’s 18th District How each county might vote if the race were tied districtwide Presidential margin County 2016 Vote Share 2012 2016 Partisan Lean Benchmark Allegheny 43% R+8 R+4 R+7 D+14 Westmoreland 33% R+27 R+34 R+35 R+13 Washington 22% R+18 R+27 R+27 R+6 Greene 2% R+25 R+43 R+41 R+19 Total 100% R+17 R+20 R+21 EVEN * The average difference between how the county voted and how the country voted overall in the last two presidential elections, with 2016 weighted 75 percent and 2012 weighted 25 percent. Source: Daily Kos Elections

Granted, the district may not shift uniformly. Westmoreland, Washington and Greene all voted much more Republican with Trump on the ticket than with Romney, while Trump seemed to scare away some Romney voters in Allegheny. Lamb’s Blue Dog appeal may mean he wins over more Obama-Trump voters than Romney-Clinton voters, making the 2016 election a poor template for 2018. In such a scenario, it’s quite possible that Lamb misses the mark in Allegheny but still wins the election by overperforming in Westmoreland and Washington, so use these benchmarks with a grain of salt.

5. The aftermath

Tuesday will likely be the last time you hear about a competitive district in the Pennsylvania 18th, but whoever emerges victorious in the special will likely have to fight hard to keep his seat in the regularly scheduled November 2018 election. How can this be? Earlier this year, the Pennsylvania Supreme Court struck down the state’s current congressional map as too gerrymandered in favor of the GOP; in February, the court imposed new district lines that are friendlier to Democrats. The new map splits most of the current 18th District between the new heavily Republican 14th District and the swingier, suburban 17th District.

If Saccone wins on Tuesday, he has said that he would seek re-election in the 14th, but if Lamb wins, he is expected to go after the 17th, even though he would have to face incumbent Republican Rep. Keith Rothfus there. Of course, the loser could decide to take a second bite of the apple in either the 14th or the 17th as well. After all these long hours of campaigning and millions of dollars spent, it’s entirely possible that, at the beginning of 2019, Conor Lamb and Rick Saccone will both be members of Congress.

Both will also have to decide their next moves almost immediately after the last drink is poured at their election-night watch parties on Tuesday. The deadline to file to run in November is March 20, one week later.

UPDATE (March 12, 2:44 p.m.): This article has been updated to add a Monmouth University survey released on Monday.