In the last two midterm elections, enthusiasm ran high on the Republican side while Democrats were dispirited. As a result, the GOP came to dominate state politics more than at any time since the 1920s.
It's likely to be a different picture this time around.
There's no guarantee that Democrats will see a "blue wave" as powerful as the GOP tides of 2010 and 2014. But it's clear that Democrats have closed the enthusiasm gap that dogged them in midterm races during the Obama presidency.
The primary season came to a close last Thursday in New York. Across the country this year, citizens cast nearly 23 million votes in Democratic primaries. That's more than the 19 million votes that were cast in Republican primaries -- and a big jump over the 14 million votes cast in Democratic primaries back in 2014. That year, 15.5 million votes were cast in Republican primaries.
Democratic turnout doubled in 14 states this year, compared with 2014, while doubling in only one state's GOP primary. That was Vermont, where Democratic turnout increased by an even greater percentage. Republican turnout was up at least 70 percent in Connecticut, Florida, Oklahoma, Pennsylvania and Wisconsin.
"Republicans have tended to show up more than Democrats, but there's a lot of enthusiasm among Democrats for remaking the composition of state and local bodies," says Edie Goldenberg, a University of Michigan professor of public policy.
Why Is Democratic Primary Turnout Higher This Year?
Democrats this year appear to be motivated in large part by the presidency of Donald Trump, whom they view with nearly universal disdain.
"Unfortunately, anger's a pretty powerful motivator, and the left is motivated -- more so by the president and others in Washington than me -- but they're motivated," Wisconsin GOP Gov. Scott Walker told Governing in an interview last month.
Trump's support among Republicans remains solid, in the mid to high 80s, according to recent Gallup polling. But there's a risk of complacency settling in among his backers.
Polling conducted for the Republican National Committee earlier this month and leaked to reporters over the weekend shows that most Republicans -- especially strong Trump supporters -- don't think Democrats can take over the U.S. House in November. In part, that's because they dismiss the findings of pollsters who predicted Trump's defeat in 2016.
"They don’t believe there is anything at stake in this election,” the GOP pollsters wrote.
Republican politicians and strategists are hoping to fire up their party's voters by convincing them that serious losses are a real possibility. In the meantime, however, they note that it's not as if Republicans aren't turning out this year. GOP primary turnout did increase compared with 2014 -- just not as sharply as Democratic turnout.
"Republicans were energized in both 2010 and 2014, so if Republican turnout is up compared to those two midterms, that's a pretty strong indicator," says GOP consultant and pollster Whit Ayres. "It's a very low bar for the Democrats -- they were disheartened in 2014."
Does Primary Turnout Matter?
Political scientists generally believe that primary turnout isn't particularly indicative of who will vote in the fall, or which party they will favor.
"We should be skeptical of reading too much into primary turnout," says Patrick Miller, a political scientist at the University of Kansas. "It's one of many tea leaves."
But it is an indication of enthusiasm. Voters are already less likely to turn out for primaries than general elections, so if one side is seeing a big bump in the primaries, it could be a good sign for the fall.
"Part of what drives presidential party losses in the midterms is that the other party is more angry and excited, and also more interested in politics," Miller says.
Primary turnout may just be one tea leaf among many, but several other indicators are also looking positive for the Democrats. They are well ahead in generic preference polls asking which party voters would prefer to see in control of Congress. They have performed well in special elections. And they've been able to field large numbers of candidates.
This year, there are Democrats running in all but two U.S. House districts, nominating the most candidates ever fielded by a party, according to Ballot Access News. At the legislative level, 5,176 Democrats are running for the 6,066 seats available, compared to 4,608 Republicans. Democrats are running for nearly every seat in states such as Georgia and North Carolina where they had left dozens of districts uncontested in recent years.
More candidates has led to more competition, which in turn draws out more voters. It's possible that there were simply more interesting races on the Democratic side than in GOP primaries.
"If one party has an interesting primary and the other has an incumbent running effectively uncontested, that might skew participation toward the party out of power," says Henry Olsen, a senior fellow at the conservative Ethics & Public Policy Center in Washington. "Since in many states that party is the Democrats, on the margin, that might skew things in their direction."
The share of the overall vote cast this year in Democratic primaries -- 53 percent -- is roughly in line with that taken by Republicans in 2010 (56 percent) and 2014 (55 percent) and by Democrats in 2006 (54 percent), according to John Couvillon, a GOP consultant in Louisiana. All of those midterms ended up being considered "wave years" for the party that had dominated primary voting.
"The signs of stronger commitment to vote by Democrats suggests to me that there will be a closing of the gap between Republicans and Democrats," says Goldenberg, the Michigan professor, "and indeed that Democrats may have higher turnout than Republicans."