Due to New Jersey’s new vote-by-mail law, Election Day doesn’t end Tuesday.

With the prospect of close races for Congress, and even for offices in small towns where election margins are inherently narrow, it could mean patience is required to wait for the winners. And it could mean campaign lawyers, if things are truly narrow and contested.

“We may not be able to report full, unofficial results on election night,” said Robert Giles, director of the state Division of Elections. “The thought is that it’s more important to count every eligible vote cast than it is to have speedy results that are reported.”

There are two main reasons. The first is that more than 550,000 vote-by-mail ballots were sent this year, reaching nearly one of every 10 voters in the state. Some of them requested ballots this year, but others were sent one because a state law enacted in August required one to be mailed to anyone who requested one in 2016, when the presidential election topped the ballot.

So far, more than 305,000 of those ballots have been returned, according to the Democratic voter data firm TargetSmart. Some of the others voters with mail-in ballots won’t participate in the election, as happens every year, but county election officials expect others to show up at their regular polling place. Those voters are supposed to be allowed to vote by a provisional paper ballot. Those can then take up to a week to verify and count.

That’s a standard process that simply might be used more this year. But the second reason is new: Vote-by-mail ballots that are received by county officials as late as 8 p.m. Thursday, 48 hours after the polls close, must be counted if they’re postmarked by Tuesday. In the past, anything that arrived after Election Day was labeled as too late and the voters notified their ballot didn’t count.

“I believe that it’s going to be a little dicey,” said Monmouth County Clerk Christine Giordano Hanlon. “I hope that it won’t be, but with close races, I think that there’s the potential that we will not have election results in many cases on election night.”

Counties operate the elections, not the state, and there’s no set rules for whether the 21 counties will announce late mail-in votes each day or wait until Friday, after all ballots that will count have been received. The board of county canvassers in each county has until Wednesday, Nov. 14, to meet to check the county clerk’s vote tally.

If races are close and campaign lawyers get involved, the state will represent county election boards and superintendents, Giles said.

“Obviously if a legal issue arises, the attorney general’s office will become involved in those offices, as needed,” Giles said.

The vote-by-mail expansion law is intended to encourage voter participation. Voters who are added to the mail-in ballot program will receive mail-in ballots for all future elections, unless they opt out in writing.

TargetSmart says that as of Wednesday, 305,480 mail-in ballots had been cast in New Jersey, with an average of about 10,000 votes arriving a day for the past three weeks. At the same point four years ago, the last congressional midterm election, 113,432 votes had been cast. In total in 2014, there were 143,094 mail-in votes.

As mail-in ballots arrive in county offices, they’re logged in to make sure that a person doesn’t vote a second time at a polling place. They’re also analyzed to ensure they’re valid, such as cross-referencing a voter’s signature and other information.

Before the new law, all mail-in ballots had to be counted on Election Day. County election boards can start tallying them when the polls open in the morning, and often the mail-in results are the first posted online when the voting ends at 8 p.m.

Paper provisional ballots aren’t looked at until after the polls close. The superintendent of elections offices then verify those ballots were cast by registered voters and that the same person hadn’t voted someplace else in the county or by mail. That can take a week or more, before the ballots are counted and added.

Election night results are always unofficial. In last year’s governor’s race, an additional 141,000 votes, or 7 percent of the total turnout, were added to the unofficial tally after election night.