The claim that an upcoming election "all comes down to turnout" is one of the most venerable clichés in American punditry. But it's become more popular than ever in an era when the mass public is commonly characterized as consisting of two implacable partisan tribes, equally unshakable in their preferences and aversions. If virtually every potential voter is a loyal member of either the red team or the blue team, the outcome of a national election is presumably determined simply by which party can motivate its supporters to participate at the highest rate.
Swing voters are indeed less numerous than they used to be, and the geographic polarization of the American electorate has reduced the number of states and congressional districts that are politically competitive in any given contest. But at a time when the two major parties are closely matched in national strength, the voters who remain open to persuasion continue to hold a lot of electoral power. And it's far from clear whether there will be a large enough difference in the participation rates of committed Democrats and Republicans for turnout to be the primary factor deciding the 2018 election.
To be sure, evidence is piling up that Democratic voters are unusually mobilized this year compared to the recent past. A September survey by the Pew Research Center found that 67 percent of Democratic supporters reported being "more enthusiastic than usual" about voting—a much higher rate than Pew found in either 2014 (36 percent) or 2010 (42 percent). Turnout in Democratic primary elections surged to 23 million voters in 2018, up from 14 million in 2014. And the astounding fundraising totals reported by Democratic congressional candidates, fueled by an unprecedented explosion of small-dollar contributions by individual donors, surely reflects an unusual degree of engagement among politically attentive Democratic citizens—and also ensures a series of generously-funded Democratic get-out-the-vote operations from one end of the country to the other.
But 2010 and 2014 were both unusually poor elections for the Democratic Party nationwide. Improved Democratic participation in 2018 compared to the two most recent midterms may prevent another disastrous performance, but it hardly guarantees a blue wave. And while Democrats are clearly much more engaged this year than in the recent past, Republicans are not necessarily less engaged.
According to Pew, 59 percent of Republican supporters are "more enthusiastic" about voting than usual in 2018—not far behind Democrats and at least equal to Republicans' own reported enthusiasm levels in 2014 (52 percent) and 2010 (57 percent). Among respondents to a recent Washington Post poll, 81 percent of Democrats and 79 percent of Republicans claimed to be "absolutely certain to vote" in November (surely a massive collective exaggeration of the actual turnout rate, but not one that reveals a significant difference between the parties), and an NBC-Wall Street Journal survey found that 72 percent of Democrats and 68 percent of Republicans reported "high interest" in the 2018 elections. Similarly, while the GOP didn't experience a spike in participation as large as that of the Democrats this year, turnout in Republican primaries was still higher in 2018 than 2014, growing from 15.5 million to more than 19 million votes.
Moreover, few pollsters are finding that restricting their analyses to the fraction of respondents identified as certain or likely voters (as opposed to all registered voters) produces significantly more favorable results for Democratic candidates in 2018. In fact, it's relatively good news for Democrats that they don't seem to lose ground when survey analysts use a "likely voter screen" to compensate for the projected composition of this year's electorate. Republicans normally enjoy a persistently higher turnout rate in midterm elections that diminishes or disappears in good Democratic years but seldom, if ever, transforms into an actual pro-Democratic turnout advantage. As Nate Cohn of the New York Times observes, "When Democrats hold the presidency, Republicans generally have a big midterm turnout edge . . . [and] when Republicans hold the presidency, Democrats fight back to parity."
Based on the incomplete signs so far from state election officials' reported early voting and absentee balloting totals, turnout is likely to increase across the board in 2018 from its 2014 levels. Democrats, of course, are strongly motivated this year by their deep antipathy to the Trump presidency. But Republicans don't appear to be staying home either—certainly not to the degree that Democrats did during the two Obama midterms. The polarizing figure of Trump may be inspiring elevated engagement on both sides; at minimum, it seems likely that the constant public attention commanded by the current president has resulted in Americans of all partisan persuasions thinking and talking more about politics than they did before he took office.
With Democrats and Republicans both invested in this year's election, a potential nationwide blue wave will require a non-trivial proportion of voters to shift from the GOP (or third parties) in 2016 to Democratic candidates in 2018. There are 25 Republican-held House seats that were carried by Hillary Clinton in 2016, barely more than the minimum net gain (23) needed by the Democrats to take control of the chamber, and it seems unlikely that Democrats could win enough of these seats alone to gain an overall majority. But there are also 16 Republican-held seats that Trump carried with less than 50 percent of the total popular vote, 23 additional Republican seats where Trump received between 50 and 52 percent of the total vote, and another 24 seats where Trump received 53 or 54 percent of the vote. These are the pivotal districts that hold the partisan balance of power in the House. Democrats don't need to peel off a large share of voters who previously preferred Republican candidates in order to capture majority control, but merely energizing their own habitual partisan supporters is probably insufficient to flip enough seats their way absent a modicum of successful persuasion as well.
In the Senate, the Democrats' need for a lopsided advantage among swing voters is even more evident. Five of the six most vulnerable Democratic incumbents this fall were elected in 2012 even as Mitt Romney carried their states over Barack Obama (Joe Donnelly of Indiana, Claire McCaskill of Missouri, Jon Tester of Montana, Heidi Heitkamp of North Dakota, and Joe Manchin of West Virginia); the sixth, Bill Nelson of Florida, represents a state where Obama achieved a narrow plurality win. All six states shifted further toward the Republicans in 2016; these Democrats are thus dependent upon a significant share of their constituents continuing to divide their partisan preferences, and this dependence increases further with every sign of a mobilized Republican base in 2018.
According to exit polls, self-identified independents preferred Republican candidates by margins of 56 percent to 44 percent in 2014 and 59 percent to 41 percent in 2010, making an underappreciated but critical contribution to the national GOP sweep in both elections. In 2006—the last Democratic midterm victory—independents gave Democrats an equally favorable margin of 59 percent to 41 percent. Leads for Democratic candidates in 2018 voter surveys, including a persistent advantage for the party in the national generic congressional ballot, have similarly been fueled by a steady erosion of Republican support among independents since the 2016 election. The addition of these independent votes to the revved-up Democratic "resistance" seems like a formula for electoral success in November, but many persuadable voters are not as attentive to politics as strong partisans are, and their preferences are likely to be somewhat unsettled even as Election Day swiftly approaches. With so many seats in play at all levels of government, it's still too soon to tell exactly how far the swing vote will swing.