“Be patient,” advised William G. Voelp, chairman of the Maryland State Board of Elections, whose members are appointed by the governor. “Every legal vote will be counted, and then the state board of elections will certify based on not more than and not less than every legal vote being counted.”

The popularity of voting by mail shot up during the coronavirus pandemic as Maryland and other states sought to make voting safer. Maryland, however, is the only state that prohibits processing these ballots before the polls close, according to the National Conference of State Legislatures. The regulation, created in an era when mail-in, or absentee, voters were a tiny portion of the electorate, addressed concerns that mail-in results could be leaked before Election Day and sway the overall outcome.

In April, the Maryland General Assembly approved a bill to permit processing mail-in votes before Election Day, giving election workers a head start. Gov. Larry Hogan (R) supported the measure but vetoed the larger bill, saying it did not contain election security measures he favors.

The May 27 veto surprised many election officials and caught them flat-footed. The state board of elections considered asking a court to waive the rules, but as time ran short, it decided against it. By late last month, when the board made the decision, there was little time for election officials to act ahead of early voting, when processing mail-in votes is feasible, Voelp said. It’s difficult for local election officials to simultaneously process mail-in ballots and manage early voting.

Voelp, who cautioned that he couldn’t speak for the board, said he “would hope that we would seek relief from the courts for the general [election in November] in a timely enough fashion that the local boards could set up for that and handle that.”

While the 2020 primary and general election saw huge volumes of mail-in voting, a now-expired emergency order by Hogan during the health crisis allowed for processing mail-in votes before Election Day.

While it’s difficult to predict what proportion of voters will vote by mail in the primary, there are signs it will be significant. As of Sunday, local election offices had received just over 115,000 mail-in ballots. Nearly 500,000 people had requested mail-in ballots. Of these, 35 percent came from voters who requested them specifically for the primary or for both the primary and general elections, election officials say. The rest were from voters who asked at some point to be “permanent absentee voters.”

In Maryland’s 2018 primary, which like this year had no presidential race, 872,207 people — 24 percent of eligible voters — voted. Of those, 30,122 cast mail-in ballots.

Delayed results in tight races have long been the norm. In 2018, it took two weeks to declare the winner of Montgomery County’s Democratic primary for county executive, which came down to absentee and provisional ballots. Marc Elrich saw his Election Day lead drop with each new round of ballots counted. In the end, he won by just 77 votes.

This year, with a high volume of mail-in ballots awaiting tabulation, candidates will need more-decisive margins among in-person voters to declare victory on Election Day.

“I don’t know really what to expect, except that I think it’ll take a while, and I think that people will be impatient because they want to know the results,” said David A. Naimon, secretary of the board of elections in Montgomery County, which has the largest number of eligible voters of any jurisdiction. “We’re going to do it as efficiently as we can, but obviously the most important thing is that we do it accurately.”

Many officials and observers are concerned about the possibility that a candidate could take advantage of delayed results to claim fraud. Former president Donald Trump did this in the 2020 election; in several states, he led in-person voting tallies, only to have his lead later wiped away by mail-in ballots. Trump falsely claimed the mail-in votes were invalid.

Though Trump’s claims of election fraud were backed by no credible evidence and comprehensively rejected by judges across the political spectrum, they continue to undermine confidence, particularly among Republicans, in the integrity of elections.

One of the candidates in the Republican primary for governor, Del. Daniel L. Cox (Frederick), is endorsed by Trump and has perpetuated his election-fraud claims. Cox, following like-minded election-result deniers in states such as Arizona and Pennsylvania, is calling for a “forensic audit” of the 2020 results in Maryland, where Joe Biden won with 1,985,023 votes against Trump’s 976,414.

Cox did not return an email or phone message on Monday. A recent Goucher College poll, conducted in partnership with the Baltimore Banner and WYPR-88.1, found him in a statistical dead heat against his opponent, Kelly M. Schulz, among decided Republican voters.

“Any time you have delays, it sows these folks who want to come out and talk about conspiracy theories, and they want to inject some doubt in our system,” said Kevin Kinnally, legislative director of the Maryland Association of Counties. Such claims breed threats and intimidation of election workers, among other problems, he said.

“Mostly what we’re trying to do is make sure that people have the correct expectations here,” Kinnally said. “That they shouldn’t expect all the results on election night, that they may have to wait some time.”