More than 40 million Americans voted in primaries this year, a staggering increase from four years ago and a sign of virtually unprecedented voter enthusiasm ahead of the midterm elections.

Primary voter turnout was higher than in 2014 for both Democrats and Republicans in most states across the country — though Democrats have a decided advantage.

Through Thursday’s vote in New York, more than 22.7 million Democrats had cast ballots in party primaries, compared with just 13.8 million in 2014.

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Among Republicans, 19.3 million showed up to vote, an increase from the 15.5 million who voted in GOP primaries four years ago.

Political scientists say the higher turnout among primary electorates is a sign that voters across the spectrum are more excited to take part in the midterms than in previous years. Some said higher participation in Democratic primaries should worry Republicans, who already face a challenging midterm cycle.

“The surge in Democratic primary turnout shows that the party’s occasional voters are energized, which is an especially encouraging sign in a midterm because so many of these voters sit out anything but a presidential race,” said Thad Kousser, who heads the political science department at the University of California-San Diego.

Comparing turnout between different midterm years is imperfect at best. Six-year Senate terms mean voters in some states do not have a reason to show up to the polls in a primary, an off-year or uncompetitive governor race, where an incumbent might face only token opposition.

Democrats saw the biggest increase in turnout in Delaware, for example, where a competitive race between Sen. Tom Carper Thomas (Tom) Richard CarperThe conservative case for phasing out hydrofluorocarbons Democrat asks for probe of EPA's use of politically appointed lawyers Overnight Energy: Study links coronavirus mortality to air pollution exposure | Low-income, minority households pay more for utilities: report MORE (D) and his liberal challenger drove voters to the polls this year; four years ago, Delaware Democrats renominated Sen. Christopher Coons Christopher (Chris) Andrew CoonsMurkowski: Supreme Court nominee should not be taken up before election Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE (D), who faced no serious primary challenge.

"One reason we're seeing higher turnout among Democrats in 2018 than in 2014 is that 2018 promises to be a good electoral environment for Democrats, so more candidates are contesting party nominations," said Michael McDonald, a political scientist at the University of Florida. "Voters are more likely to participate in a competitive election than a noncompetitive one."

And some states do not bother to hold primary elections for uncompetitive races. In New York on Thursday, nearly 1.5 million Democrats voted in the race for governor, almost a threefold increase over the 2014 contest. But no Republican votes were recorded, because Dutchess County Executive Marc Molinaro ran unopposed for the GOP nomination.

Correcting for those states where one side faced a primary and the other didn’t, Democrats still managed to turn out more than Republicans by a nearly 2 million-vote margin. In 2014, the situation was reversed: 2.2 million more Republicans voted in party primaries than did Democrats.

"When the primary turnout becomes anomalous, and suddenly it's surging in one direction or another, that does tend to have some bearing on general election turnout," said Tom Bonier, a Democratic microtargeting expert. "Democratic turnout pretty consistently surged over Republican turnout."

Some of the states where Democratic turnout increased the most are states at the heart of the battle for control of Congress, a potentially worrying sign for Republicans trying to hold on to their fragile House majority.

Democratic turnout more than tripled in Minnesota, where the parties are fighting over two Democratic-held seats in rural areas and two Republican-held seats in the Twin City suburbs. Both sides handled competitive gubernatorial primaries, and Republican turnout rose too, by about 74 percent.

All told, 582,000 Minnesota Democrats cast primary ballots, compared to 320,000 Republicans; four years ago, just 191,000 Democrats and 184,000 Republicans voted in party primaries.

Democratic turnout more than doubled in 14 states, including in House race hotbeds such as Colorado, New York, Iowa, Kansas, Virginia, Michigan, New Jersey and Nevada.

Republican turnout doubled in only one state, Vermont, where Gov. Phil Scott (R) survived a long-shot challenge from a conservative activist, taking 65 percent of the vote. Democratic turnout in the state rose 163 percent, and the party’s gubernatorial nominee, former Vermont Electric Cooperative CEO Christine Hallquist, took about 2,600 more votes than Scott in his.

Republican turnout increased by more than 90 percent in Pennsylvania, which had a competitive gubernatorial election, and Wisconsin, where two well-funded candidates fought over the right to face Sen. Tammy Baldwin Tammy Suzanne BaldwinSenators introduce bipartisan bill to mandate digital apps disclose country of origin Keep teachers in the classroom Cher raised million for Biden campaign at LGBTQ-themed fundraiser MORE (D) in November.

GOP turnout rose 80 percent in Connecticut, and by 70 percent or more in Oklahoma and Florida, all three of which had contested gubernatorial primaries.

Kousser said voters who show up in primaries are almost certain to show up again in November.

“Voting is habit-forming,” Kousser said. “So turning out in the spring makes these voters more likely to participate in the fall.”

Overall, turnout in Democratic primaries increased in 37 of the 47 states that held comparable contests in 2014 and 2018. Most states where turnout dropped — like Kentucky, North Carolina, Arkansas, Alaska and South Dakota — did not feature any competitive contests near the top of the ticket.

On the GOP side, turnout increased in 34 of 46 states that held comparable contests in the two midterm years. Republican turnout dropped in some that featured competitive statewide primaries, like Arizona, Mississippi, Illinois and New Hampshire, where both of the state’s two districts featured competitive Republican primaries.

McDonald said the increase in turnout is likely to be "unusually high" in November.

Bonier, the Democratic strategist, said his party should pay attention to higher turnout among Republican voters. Unlike in 2006, when Republican voters were depressed enough to stay home, today's GOP is more excited than it typically is for midterm elections.

"The one reason why Democrats shouldn't be doing backflips yet, it's not that Republicans are depressed, they're just not surging at the level that Democrats are," Bonier said. "They appear to be surging above levels that are typical of midterm turnout."

Still, high primary turnout is another data point that hints at a strong year for Democrats. Several recent surveys testing the generic ballot matchup between unnamed Democratic and Republican candidates, from outlets like CNN, Quinnipiac University, Marist College and Emerson College all show Democrats leading by double-digits. A Reuters/Ipsos poll this week pegged the Democratic edge at 8 points.

President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE looms large over Republican hopes in November as well. Trump’s approval rating is dismally low, ranging from the high-30s in the CNN poll to the low-40s in the Reuters poll.

Significantly more voters, 58 percent, told Quinnipiac pollsters they want Congress to be more of a check on Trump’s agenda than the 27 percent who said Congress was doing enough.