MINNEAPOLIS — Volunteers brought the coffee and bagels. More than a dozen poll workers sat attentively, ready to handle an onslaught of voters. And Tom Knabel got in line at 5:30 a.m. to make sure he was first to cast his ballot for president of the United States.

He was early. Forty-six days early, to be exact.

This election — a dead-heat battle between two highly unpopular candidates — officially started here Friday as Minnesotans, at 8 a.m. under cloudy skies, began casting their ballots in a government building in downtown Minneapolis.

Early voting is getting earlier for many Americans, transforming what was once a singular Election Day into an election season spanning months. It’s an escalating trend: In-person voting also started statewide Friday in South Dakota and Vermont, and even earlier this week in parts of Wisconsin.

Never mind that voters were filling in ballots days before the first of three presidential debates between Hillary Clinton and Donald Trump. Or that summer is barely over and the leaves haven’t changed.

“Being from Minnesota, I’m used to voting when it’s cold,’’ said Knabel, who showed up with his backpack filled with crackers, cheese, and water to sustain himself for the 2½ hours before the polls opened. “Voting when it’s 70 degrees is strange in Minnesota. It just is.’’

Voters in 37 states, as well as the District of Columbia, are allowed to vote early, and it is changing the complexion of the political contests by giving campaigns more opportunities to ensure their supporters turn out.

And what used to be just mail-in ballots — and only for those who had a valid excuse — is now also in-person voting, and with no excuses needed.

The upshot is that instead of a 72-hour get-out-the-vote effort aimed at delivering people to the polls on a Tuesday in November, it’s a weeks-long tactical slog. Massachusetts begins early voting on Oct. 24, part of a law signed in 2014.

In the 2000 election, about 15 percent of voters cast their ballots early, while about a third did so in 2012. This year it could climb as high as 40 percent.

In Minnesota, a solidly blue state, early voting likely will not affect the outcome. But voting will begin in major battlegrounds, such as Arizona and Ohio, on Oct. 12, with North Carolina and Florida shortly after.

Some worry about having voting take place so early because an election can change dramatically in 45 days. And while voters can technically go back and retrieve their ballots and change their early votes until the week before the election, few do.

“A lot happens in elections in those last few weeks,’’ said Keith Downey, the chairman of the Minnesota Republican Party. “One set of people are voting with one set of information, and another set are voting with a different set of information.’’

Early voting tends to reward the better-organized campaign, which can ensure supporters cast their ballots. Advantage: Clinton.

Results are not known until the final Election Day — this year, that’s Nov. 8 — but campaigns receive data that show who voted each day, allowing them to track down laggards and urge them to perform their civic duty.

It can have a demonstrable impact: In 2012, President Obama banked a large number of votes in Iowa before the election and had a lead that was sizable enough to win the state, even though Mitt Romney got the most votes on Election Day itself.

“The parties are changing their strategies,’’ said Michael P. McDonald, an associate professor at the University of Florida who studies the electoral system.

The Obama campaign, for example, would hold events near voting precincts, so after a rally on a college campus, students could go cast a ballot. And once they felt like they had an insurmountable lead based on early voting, they could shift resources to other states.

“The Democrats in 2008 were really the first ones to put an emphasis on early voting as their voter-mobilization strategy,’’ he added. “Republicans started catching up in 2014 with key battleground states like Iowa.’’

Those who cast ballots early tend to be well-educated, reliable voters who follow politics closely. In liberal Minneapolis, where about 146 voters had cast ballots by midday, there was no visible sign of the Trump campaign.

“There’s something about the experience of being in line on Election Day, which I miss by doing this,’’ said Steven Girshick, a 67-year-old mechanical engineering professor. “On the other hand, some 100 million people will vote. And I’ll be one of the first. There’s something special about that.’’

Some said they would still watch the debates, but nothing that will be said would change their minds. Others are happy to have done their civic duty; now they can tune the election out.

Until fairly recently, early voting — typically by absentee ballot — was somewhat rare, restricted to soldiers, college students, and those living abroad. But it began expanding a decade ago, the change motivated by a desire to boost turnout by making it easier to vote.

Increasingly, partisanship has crept into the equation. Republicans have raised questions over the potential for fraud, although there are few examples of it, while Democrats have pushed to make it easier for parts of its coalition — young and minority voters — to cast a ballot.

In some states, the election can essentially be over before Election Day. In several battlegrounds, including North Carolina, Florida, and Nevada, more than half the votes will be cast before Nov. 8, election officials predict, by both in-person early voting and voting by mail.

Clinton’s campaign organized a small pre-vote rally in Minneapolis on Friday morning, with about 30 people arriving by bike, car, and foot.

“It matters, because in our history we’ve never had a woman president,’’ said Corinne Ertz, who brought her 13-year-old daughter. “It’s time.’’

Inside, about 15 polling workers sat at tables filled with hand sanitizer, coffee, and ballots.

“I love it,’’ said Pam Moret, a 60-year-old recent retiree. “If it were earlier, I’d vote earlier.’’

Voting was light. Very light. One polling center — in an area that normally has high turnout — had only two people show up over the course of 90 minutes; only one was eligible to cast a vote (“She was a good voter, though,’’ a city worker said).

Late Friday morning, Meg Forney came into a voting center. She took selfies inside the voting center before casting her ballot, and afterward she grabbed several “I voted’’ stickers and affixed one to her shirt.

“I’m going to keep walking around for the next 47 days with a sticker,’’ she said. “To remind people this is important.’’

Matt Viser can be reached at matt.viser@globe.com.