Political talkers who refer to a "wave," as in "a blue wave is coming," never define the term. Sometimes, listening to them, it seems to mean nothing more than that the Democrats are poised to win a majority of seats in the House of Representatives. But in that case it seems the term is just more generic hyperventilating. After all, the "out party" picks up, on average, about 30 House seats in a midterm election, and the Democrats only need to gain 23 to win the majority.
Wave signifies wipe-out, right? Like what President Obama called "a thumping" after Republicans gained 63 seats in the 2010 congressional election. That was a "wave," or a "tsunami." Just for purposes of having some kind of concrete marker, let's say that Democrats will have ridden a wave if they gain more than 40 seats in the House and, despite a very unfavorable map, win a majority of seats in the Senate as well. What is the prospect of that happening?
Since there is today a special election for the twelfth congressional seat in Ohio, let's leave the Senate side of it for another day. Ohio's twelfth district is, to trot out another cliché, rock-ribbed Republican. Trump carried it by 11 percentage points, which was not a particularly strong performance by a Republican. The district has been represented in Congress by a Republican for more than 30 years. Today's special election was triggered by the retirement of Republican Pat Tiberi, who was first elected in 2000. He was then re-elected eight times, with an average of more than 55% of the vote. In three re-elections since the district was rearranged into its current configuration after the 2010 census, he has averaged 66% of the vote. According to the "partisan-lean" index of the FiveThirtyEight website, Ohio-12 is 14 points more Republican than the country as a whole.
As can be seen in the above maps, the district is shaped like a sideways J, with the city of Columbus rubbing against the outside of the curve. The part of the district in northern Franklin County, home to Columbus, is strongly Democratic--Hillary carried the county, 56-38. A third of the district's voters live in Franklin County. Since Trump carried the district by 11 points, you can see what happened in the other two-thirds.
Yet everyone is expecting a close outcome tonight. The candidates are unexceptional, more or less generic representatives of their party, and the Republican, Troy Balderson, began with a double-digit lead in sporadic polling, which, considering the district, seemed unexceptional, too. But his position seems to have steadily eroded as the election drew near, until, in a poll conducted by Monmouth University in the last week of July, his lead stood at a statistically insignificant 47-45. The last poll there will be, before the final one tonight, is the only one showing the Democrat, Danny O'Connor, the 31-year-old county recorder of Franklin County, with a lead: 47-46. That was an Emerson College poll conducted August 2-4.
Perhaps the dramatic narrowing of this race in a Republican district is not that surprising. Trump easily carried Ohio, but his approval rating in the state has slipped from +14 when his term began to -4 in May of this year, the most current data point I can find. Many observers think Balderson was likely hurt more than helped by Trump's rally in the district last week. FiveThirtyEight observes that, in federal special elections conducted this year and last, Democrats have outperformed their partisan-lean index for the election venue by an average of 16 points. Recall that the partisan-lean index for Ohio-12 is Republican +14, not quite enough to sustain a 16-point erosion.
Circling back to the original question concerning the prospect of a "blue wave," it hardly matters whether the Republican is upset or hangs on to win by a point or two. If O'Connor wins, the Democrats' "magic number" will slide from 23 to 22. But for crystal ball purposes, the alarming figure for Republicans must be the number of congressional districts that have a Republican representative despite being less Republican than Ohio-12, which in 2018 appears to qualify as a "swing district." I heard on the news this morning that, according to the Cook Political Report, that number is not less than 60. Of course there are the normal caveats: special elections are "special," there's no incumbent, the actual election is two months off, the national mood could change, etc., etc. If, however, we set a gain of 40 seats as the mark for a Democratic wave in the House, it's now easy to see from shore something that looks . . . wave-like.