It's always possible to overinterpret the outcome of a single special election; at the same time, even one more data point can help us make a little bit more sense of the political world around us. Here are a few things that can be gleaned from tonight's results in Pennsylvania:
1. The photo finish between Democratic candidate Conor Lamb and Republican nominee Rick Saccone (with Lamb currently in apparent position to eke out a victory) in a district previously considered safely Republican indeed represents a notable development, but not a shocking one. It comports with the historical pattern of electoral politics: the opposition party reliably makes gains in midterm elections, and the magnitude of the swing is correlated with the (un)popularity of the president. The importance of tonight's outcome lies primarily in its confirmation that these dynamics still hold today as they have in the past. But the current state of President Trump's job approval rating and the congressional "generic ballot" polling already signaled that 2018 is likely to be a good year for Democratic candidates.
2. Even so, the outcome in PA-18 may itself prompt members of the Washington community to revise their predictions about what's likely to happen in November, although there's plenty of existing information on which to base their analyses. There's an undeniable psychological difference between expecting something to happen and actually watching it occur. I remember the 2006 midterms, when—despite piles of survey data pointing to a Democratic takeover of the House of Representatives—a number of pundits had a hard time actually envisioning an end to what at that point was a 12-year Republican reign over the House until the votes actually came in on the night of the election.
As for the parties themselves, whatever spin we hear from either side doesn't mean very much. What's important is what they do. Will Republicans start to act as if they believe their majority is now in serious danger? Will there be criticism directed, on the record or on background, by rank-and-file members of Congress toward Republican leaders, including the president, for putting the party in such a precarious position? Or will incumbents sincerely adopt the view that the results in PA-18, along with those in the Alabama Senate race last year, reflected a mismatch in candidate quality more than a fundamental deterioration of the GOP's electoral strength?
3. Special elections can also be opportunities for the parties to test out their campaign messages in advance of a national vote. One lesson that the Republicans appear to have taken away from their Pennsylania experience is that the December tax cut bill is of limited popularity and/or salience in the electorate. Since the current congressional majority has few other accomplishments for which to claim credit, this suggests that the fall elections will be fought over something other than the legislative record of the past two years. (It's likely at any rate that the midterms will be dominated by Trump and Trumpism regardless of what most individual candidates do.)
4. It's common for election analysts these days to use the 2016 presidential election results, or measures derived from them (such as the Partisan Voting Index, or PVI), as a benchmark to characterize the party leaning of states and congressional districts: a +5 Clinton seat, a +10 Trump seat, etc. In general, presidential and congressional voting results are strongly correlated, making these figures good rules of thumb in most cases. But there are still parts of the country where voters behave somewhat differently when choosing presidential and non-presidential candidates; moreover, the distinctive candidacies of Hillary Clinton and Donald Trump (and, before that, Barack Obama) have the potential to produce slightly misleading pictures of the "fundamental" partisan composition of a particular constituency.
Specifically, it's worth noting that while PA-18 gave Trump a 20-point margin over Clinton in 2016, Democrats still slightly outnumber Republicans in the district's party registration figures. Washington County, most of which lies within the district, supported Trump over Clinton by 60 percent to 36 percent and Mitt Romney to Obama in 2012 by 56 percent to 42 percent, but had narrowly preferred John Kerry to George W. Bush in 2004 and had given Al Gore a 53-44 advantage over Bush in 2000.
Of course, things have changed since 2004, and recent elections are more predictive of future outcomes than more distant ones. But it's worth keeping in mind that the world of American politics did not experience a complete rebirth in 2016, rendering all previous history irrelevant. With more time and perspective, we'll be better able to tell how much of the 2016 alignment represents a "new normal" and how much is a temporary deviation from longer-term patterns.
The demonstration that a Democratic candidate like Lamb could win back many former supporters, especially among the white non-college population, who had defected in 2016 is good news for the party. Democrats are defending multiple Senate seats in states that not only supported Donald Trump over Hillary Clinton, but also shifted significantly in the Republican direction in 2016—including Missouri, West Virginia, North Dakota, Indiana, Montana, and Ohio. In order to avoid a devastating series of defeats in Senate races this year, Democrats need to attract voters who had previously backed the party's candidates but who either abandoned Clinton for Trump or merely stayed home.
At the same time, I'd recommend being a bit wary of the claim, oft-repeated during Tuesday night's coverage, that there are more than 100 Republican-held House seats that are more electorally vulnerable than PA-18. Again, that's true if we use PVI or Trump's 2016 margin over Clinton as the sole measure of partisan competitiveness, but PA-18 has more of a Democratic tradition—and labor union presence—than most other districts that gave Trump (and Romney before him) comparable margins.
Republicans undoubtedly appear to be in serious danger of losing their 24-seat (now, perhaps, 23-seat) House majority later this year. But any implication that the number of seats gained by Democrats in November could approach triple figures is not exactly realistic.