Vote-by-mail ballots are shown in sorting trays on Aug. 5, 2020, at the King County Elections headquarters in Renton, Wash., south of Seattle. (AP Photo/Ted S. Warren, File)

If, during the wall-to-wall coverage of the raid on Mar-a-Lago, you missed the primary in Hawaii, don’t feel bad. The results weren’t that interesting—at least not at first glance.

The blue state’s senior U.S. senator, Brian Schatz, crushed his underfunded primary opponent with 93.6 percent of the vote on August 14. He will likely cruise to victory over the Republican nominee, Brian McDermott. In the gubernatorial contest, popular Lieutenant Governor Josh Green defeated six Democratic competitors and is favored in the general election over former Lieutenant Governor Duke Aiona. Hawaii has had only one Republican governor since 1962.

There is one result from the Hawaii primary, however, that is worth considering: Despite how boring it was, it saw 40 percent of the state’s registered voters cast ballots. That is the third-highest turnout rate in the country this year. It is exceeded only by Wyoming’s high-profile Liz Cheney repudiation contest and the Kansas primary, which featured an abortion referendum that drew national press and finance.

To put Hawaii’s turnout rate in perspective, consider New York State’s primary on July 1. Though similarly dull (Chuck Schumer ran unopposed), it featured a spirited Democratic contest for governor, which the appointed incumbent, Kathy Hochul, won. (She will face a serious challenger, Republican Congressman Lee Zeldin, in November.) But only 11 percent of the state’s registered voters cast ballots, the lowest turnout rate of any state.

What might explain this 29-point difference in turnout between the two non-battleground blue states? One factor might be that congressional and down-ballot primaries in New York were postponed until August because of delays in redistricting. But their absence from the July primary probably accounts for only part of the low turnout.

The more likely explanation lies in how the two states run their elections. Hawaii is a full “vote at home” state, which means that all registered voters receive a ballot in the mail weeks before the election, which they can fill out and deliver either by mail or in person at a polling place or a secure drop box. New York, by contrast, has one of the nation’s most restrictive vote-by-mail regimes. Voters must provide a legally valid excuse, like a medical condition, to receive an absentee ballot.

This year, vote-by-mail policies affected participation not just in New York and Hawaii but in primaries around the country. Those turnout results, as well as new research on vote by mail’s impact on the 2020 general election, provide strong evidence that mailing ballots to voters boosts participation—and offers subtle clues of what might happen in this November’s midterms.

Take a look at the chart below, compiled by the National Vote at Home Institute (NVAHI), a nonprofit research organization. (I am one of the organization’s cofounders.) It examines turnout rates and vote-by-mail rules.

As you can see from the left side of the chart, six of the 10 states with the highest turnout are vote-at-home states (defined as states, like Hawaii, that mail every registered voter a ballot or where 75 percent or more of voters cast absentee ballots). The other four high-turnout states don’t require voters to show cause to receive an absentee ballot. By contrast, 8 of the 10 states with the lowest voter turnout rates (right side of the chart) require voters to have an excuse before being mailed a ballot, and none are vote-at-home states.  

There is plenty of “noise,” as the social scientists say, in this data. That’s because other factors besides vote-by-mail policies can affect turnout. The presence or absence of high-profile races is one. Another is the type of rules under which primaries are held. In states with “closed” primaries, where voting is confined to party registrants, participation tends to be lower than in states that allow any registered voter, regardless of affiliation, to participate in any party’s primary.

Even with these caveats, the NVAHI data suggests that vote-by-mail policies strongly affect turnout. Washington State, a full vote-at-home state, had a U.S. Senate race but no gubernatorial contest in its August 2 primary. Yet it tied Hawaii with 40 percent of its registered voters participating. Oregon, another full vote-at-home state, was not far behind, with a 38 percent voter turnout despite having a closed primary.

Then there is Nebraska, where counties with fewer than 10,000 residents can decide whether to conduct full vote-at-home elections. In the 11 Nebraska counties that do so, turnout rates averaged a whopping 55 percent, versus 32 percent in the 81 counties that opted for more restrictive vote-by-mail rules.

Since turnout in primary elections is typically low, it’s not too surprising that making it easier for voters to cast ballots by mail could yield double-digit increases in participation. But what about elections where vastly more voters participate, such as in presidential years? You’d expect the effect to be lower. The question is, how much lower?

In April 2021, in a New York Times column, the data guru Nate Cohn argued what has become a widespread view among political journalists and election analysts: Vote by mail had little to no impact on turnout in the 2020 presidential election:

There’s essentially no evidence that the vast expansion of no-excuse absentee mail voting, in which anyone can apply for a mail absentee ballot, had any discernible effect on turnout in 2020. That shouldn’t be a huge surprise: Even universal vote by mail, in which every registered voter is automatically sent a mail ballot (as opposed to every voter having an opportunity to apply for one), increases turnout by only about 2 percent with no discernible partisan advantage.

But since Cohn made that argument, two political scientists have published a study offering robust evidence that providing voters with mail-in ballots without conditions dramatically improves presidential election turnout. In the first peer-reviewed academic study of vote by mail and the 2020 election, Eric McGhee of the Public Policy Institute of California and Mindy Romero of the University of Southern California’s Price School of Public Policy examined mail ballot policies using county-level data, among other methodologies. They found that states which mailed a ballot to every registered voter in 2020’s presidential election saw voter turnout increase by an average of 5.6 percent. Turnout was even higher among infrequent voters, who tend to be disproportionately younger and nonwhite. Their conclusion: “Our results suggest that making it easier to vote by mail—especially mailing every voter a ballot—generally does increase turnout, both before and during the 2020 election.” 

As the journalist Steven Rosenfeld noted earlier this year, while the McGhee-Romero research looked at turnout as a percentage of registered voters, the studies Cohn cited generally calculate turnout against a larger population of all eligible citizens, including unregistered voters. Even so, after adjusting for this difference, the McGhee-Romero analysis concluded that mailing all voters ballots increased turnout by roughly double the estimate Cohn cites.

Perhaps their most significant finding is that mailing ballots to individuals is the form of vote by mail that most increases turnout. If voters must request a mail-in ballot, that’s a nontrivial obstacle, even if no excuse is required. In systems that raised other hurdles, like requiring that voters prove they have a disability or provide identification before receiving a ballot, turnout improves little, if at all.

Does vote by mail help one party more than another? Donald Trump thinks so. He warned in 2020 that if the United States switched to all-mail voting, “you’d never have a Republican elected in this country again.” The McGhee-Romero study found, to the contrary, a slight advantage for Republicans in the full vote-at-home states in 2020. That finding, however, may reflect that nearly all of the full vote-at-home states—Oregon, Hawaii, and so on—are blue and were not battlegrounds in 2020; hence the national Democratic Party and its donors spent relatively little time or money getting out the vote. Past research has found that voting by mail doesn’t naturally advantage either party.

But that doesn’t mean it can’t be used to partisan advantage. In 2020, New Jersey, which was not a battleground state, mailed every registered voter a ballot and saw turnout among 18- to 29-year-old eligible voters hit 67 percent, the highest in the nation and 10 to 15 points higher than in battleground states such as Michigan, Pennsylvania, and Georgia. Last year, Republican Glenn Youngkin won the Virginia governor’s race in part by convincing older, white GOP voters to ignore Trump and vote by mail. Whatever vote-by-mail system a state has can be used by parties and campaigns to their advantage.

This year, as the Washington Monthly has reported, a raft of Democratic states on the Eastern Seaboard have made voting by mail easier, even as several GOP-controlled states in the South and Midwest have made it harder. The net effect these changes will have on the midterms is anyone’s guess, and the more important factor might be what each party is doing to take advantage of the rules.

Looking back at the NVAHI chart, you will see that most states that will decide control of Congress—Ohio, Pennsylvania, Georgia, Arizona, Wisconsin, and North Carolina—are “no excuse.” That is, voters don’t need an excuse to vote by mail, but they must apply to have a ballot mailed to them.

At least some progressive groups are trying to encourage voters to do just that. The nonprofit Deliver My Vote, for instance, has digital and door-knocking campaigns in five swing states aimed at getting lower-propensity voters, including people of color and the young, to apply for absentee ballots. It also instructs them on the finer points of filling out the ballots (such as the need to sign both the ballot and the envelope in which you send it back).

How many resources the Democratic Party and its funder networks are putting into such activities is unclear. A bigger mystery is what, if anything, the GOP is doing to encourage its supporters to vote by mail. Despite Youngkin’s example, many GOP nominees in this cycle are aping Trump’s anti-vote-by-mail bombast. That could hurt GOP turnout in states like Arizona, where voting by mail has long been the norm, including among Republican voters.

It is increasingly clear that vote by mail boosts election participation and that whichever party does the most to convince its voters to utilize mail ballots will have an edge in November.

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Paul Glastris is the editor in chief of the Washington Monthly. A former speechwriter for President Bill Clinton, he is writing a book on America’s involvement in the Greek War of Independence.