WASHINGTON — Not so many months ago, casting a ballot by mail was a topic reserved for conferences of election administrators, a matter of voting mechanics blander than a water cracker. In Republican Arizona and Democratic Oregon as well as many other states, vast numbers of citizens not only voted by mail, but also loved it.

That was before the mail ballot became seen as an essential element for voting in a pandemic, and before President Trump weaponized mail voting with largely invented allegations that it would lead to massive voter fraud — despite being used for years in Democratic and Republican states without controversy.

Republican opposition seemed driven by the conviction that an increase in mail voting would benefit Democrats, who have tended to use mail ballots less compared with Republicans. But, like a lot of assumptions about voting, the reality is far less clear.

Here is a quick look at some issues surrounding voting by mail that may help to shed light on which party benefits most from it.

Are mail ballots an invitation to massive fraud?

Voting by mail is almost certainly the safest way to vote — by far — if the pandemic persists, as is likely, into November.

Is it totally free from fraud? No. In 1997, an ex-mayor of Miami won another term by paying a small army of fixers to generate false absentee ballots. More recently, the 2018 election for a congressional seat in North Carolina was invalidated and had to be run again after a worker for the Republican candidate in one county collected hundreds of absentee ballots, including blank ones, from local homes.

The catch, as those cases suggest, is that such schemes are incredibly hard to pull off undetected. A fraud big enough to swing any but the very closest elections is easily caught by looking for statistical outliers in vote totals, checking signatures and conducting basic detective work. Carrying an entire state election by mail ballot fraud would be a nigh-impossible triumph of concealment in all but a harrowingly close contest.

Elections in Colorado, Washington and Oregon, the three states that have primarily used voting by mail for the longest time, have seen no significant increase in fraud.

Do mail ballots give either political party an advantage?

Conventional wisdom in both parties is that a surge of mail ballots, such as what we are likely to see in November, benefits Democrats more than Republicans.

The logic goes like this: Traditionally, most absentee ballots were cast by Republicans, so a big increase would disproportionately help Democratic turnout. And because turnout had always been higher among wealthier, better-educated voters who tilted Republican, anything that made voting easier was bound to benefit Democrats.

But recent demographic shifts in the electorate cast doubt on that: Since Mr. Trump’s election, more educated and wealthier voters have trended Democratic, while Republicans have gained among lower-income voters, especially white people. So conventional wisdom may no longer apply.

A new working paper by the Institute for Economic Policy Research at Stanford University concluded that mail balloting modestly increased voter turnout but that both parties benefited more or less equally from the surge. Other academic studies have reached largely similar conclusions.

The Stanford study seemed especially solid because it looked at mail balloting as it was being gradually rolled out in three states — California, Utah and Washington — allowing researchers to compare counties that voted by mail with counties in the same states that did not.

But the Stanford researchers took pains to say that their findings applied to “a normally administered in-person election.” Voting in a pandemic might produce different results, they said, in part because the threat of illness might deter one group of voters from casting ballots more than it would another.

Here are two ways voting by mail could hurt Democrats.

A first question is how voters would get ballots.

Generally, the process begins when election offices mail ballots, or applications for them, to their lists of registered voters, who must have current addresses to be included on those lists. But some groups of voters — the young, minorities and the poor — relocated more often than others and would be less likely to receive ballots if they did not update their registrations.

That may be doubly true now, when economic upheaval has forced many people to move in with relatives, take in boarders or otherwise leave their usual homes. Logic says those frequent movers more often than not vote Democratic.

“The single biggest factor in determining whether someone is going to have trouble voting is whether their voting record is out of date,” David J. Becker, the director of the Center for Election Innovation and Research, said. “Mobility is the biggest factor in that, and it’s true that mobility is higher among the young and the poor.”

In many states, in-person voters can update their registration on the spot at their polls. Mail balloting offers no such escape hatch for voters, often Democrats, who have not signed up or have not kept their registrations current.

A second question is whose mail ballots are counted once they are sent in. Unlike in-person voting, where every choice is automatically toted on a machine once a voter signs in, a small but significant share of mail ballots are rejected for errors — wrong signatures, wrong dates, erroneously marked choices and so on.

A new study of Florida elections indicates that rejection rates can vary wildly from county to county, even though the standards for accepting mail ballots are the same statewide. How strictly county election officials enforce those standards, how clear ballot instructions are and how familiar voters are with casting ballots by mail all affect whether a voter is turned down, said Daniel A. Smith, a political-science professor and expert on mail ballots at the University of Florida.

In Pinellas County, Fla., which encompasses St. Petersburg, the Republican election supervisor stresses simplicity and clarity in mail ballots. In elections covered in the study, the county’s rejection rate for mail ballots was comparatively tiny — one-tenth of 1 percent. But across the peninsula, in Miami-Dade County, the rejection rate in the same election was more than 2 percent, or 20-plus times as high.

In Pinellas, mail ballots were rejected at roughly the same rate regardless of the race, ethnicity or age of the voters who cast them. But in elections in 2018 in Volusia County, which includes Daytona Beach, some 5 percent of ballots cast by Hispanics were rejected. Statewide, mail ballots by minorities were more than twice as likely to be thrown out as were ballots cast by white voters. And statewide, 5 percent of ballots cast by voters between ages 18 and 20, who are newest to mail balloting, also were rejected.

The groups most frequently rejected in the study — the young, minorities, and infrequent or inexperienced voters — tend to vote Democratic. A second study of Georgia mail voters found that rejection rates were significantly higher among voters who were newly registered, young, female or minorities — again, voters who generally tilted toward Democrats.

Here’s one important way it could hurt Republicans.

A third question — and a potentially crucial one in November — is whether election officials receive a ballot after it is sent in by a voter. And there, too, the disparities are great — and in at least one case, they tend to hurt Republicans.

A second University of Florida political scientist, Michael McDonald, recently analyzed ballots cast in 69 of 88 counties in the recent primary election in Ohio, including the state’s three largest counties, which include Cincinnati, Cleveland and Columbus.

His finding: In those large (and predominantly Democratic) counties, about 1.7 percent of mail ballots were not received in time to meet the deadline to be counted. But in 66 rural counties, 4.1 percent of ballots came in too late — and in Green County, outside Dayton, 12.1 percent of ballots arrived too late to be counted. In Ohio and elsewhere, rural voters disproportionately vote Republican.

Why the disparity? In rural areas, “mail has to be sent to a central processing facility and then it gets sent back to the local election office,” Mr. McDonald said. “And we know the mail is running slower.”

What matters most? Show me the money.

For all of the complaints about fraud from Mr. Trump and other Republicans, experts say, the truth is that political campaigns of both parties love voting by mail. That is because election offices give the campaigns lists of those voters who have asked for absentee ballots. That enables parties to focus on getting them to fill out and return the ballots, instead of wasting time and money on finding people who may not even want to vote.

And that, Mr. Smith, the political-science professor, said, underscores perhaps the most important disparity in mail balloting: money. A well-funded campaign can make up for many of the inequities that are part of the current vote-by-mail regime.

“Who’s it going to advantage?” he said. “It’s going to advantage the party that has more resources.”

Nationally, at least, that is likely to be the Republicans, not the Democrats.