Washington: In 1977, a flood control measure on the ballot in Monterey, California, became what historians say was the first modern American election decided by people who voted before Election Day.

It was a strange moment even for some who participated; elections had traditionally been a kind of civic gathering, on one day.

But the practice caught on with voters, and it eventually spread from the West Coast to 37 states and the District of Columbia. Today, at least 43 million Americans have already voted in the presidential election. And when the ballots are tallied nationwide Tuesday evening, more than one-third of them will have come from people who voted early — a record.

Voting before Election Day has become so commonplace that it is reshaping how campaigns are waged, and how Americans see the race in its final, frantic days.

“The idea that one wakes up and it’s Election Day in America is actually a rather quaint idea now,” said Russ Schriefer, a Republican consultant who has worked on presidential campaigns for two decades. “It is as much as a monthlong process to draw people in. And so your advertising tactics, your messaging tactics and certainly your ground game have changed completely.”

The spread of early balloting is forging new habits that are forcing campaigns to rethink how they allocate their resources. And it tends to favour those campaigns that are more technologically sophisticated and can identify, draw out and measure its support over a longer voting period.

In Florida, a battleground state where just a few hundred votes can tip an election and victory can guarantee the White House, new behaviours are rapidly taking hold. Hispanics, who have tended to turn out mostly on Election Day, are voting earlier in much larger numbers this year, after a major Democratic-led effort to mobilise them. This is especially true among young and first-time Hispanic voters, who are just forming their voting habits and are likely to retain the practice of casting ballots early, according to those who study early voting.

That will mean that future campaigns will need to further adapt and dedicate more time and money to chasing votes up to six weeks before Election Day.

Early voting has been expanding despite the political tensions that tend to infuse any debate over voting. Republicans often resist making voting more accessible — whether by opening more polling locations in the weeks before an election or allowing voters to mail in ballots — saying that the process could invite fraud. Republican-led legislatures in Ohio and North Carolina were successful in reducing early voting compared with 2012. Democrats, who generally favour fewer barriers and greater access, have sued to block those restrictions.

For campaigns today, the availability of real-time data on who has voted and when allows a nimble operation to determine whether those likely to support the candidate have turned out and redirect resources accordingly. Hillary Clinton’s campaign has been particularly deft, scheduling concerts with Jennifer Lopez in Miami and Beyoncé in Cleveland, where Democrats have been focusing intensely on early turnout. The artists used the occasion not just to perform but to plead with fans to vote for Clinton.

For the voter, that early turnout data is now ubiquitously covered in the news media, adding a new dynamic to the campaign horse race punditry. Of course, the data is not exact: It can show how many registered Republicans, Democrats and unaffiliated voters have cast ballots from a particular area, allowing political analysts to deduce which candidate is probably ahead.

Campaigns have long had access to similar data. But with much of the information now available on state election websites all over the country, the tally of who is up and who is down in the early vote is breathlessly followed.

“This information has gone from the war room into the newsroom,” said Paul Gronke, director of the early voter information centre at Reed College. “We’ve gotten over the tipping point here in terms of the amount of coverage and attention dedicated to these numbers.”

The availability of the information has led some prognosticators to declare races won or lost. Over the weekend in Nevada, one of the state’s top political commentators said it would be all but impossible for Donald Trump to beat Clinton there given how many more registered Democrats had already cast ballots than Republicans.

The rush to call a race before it is over raises difficult questions about whether having so much public information on early voting could interfere with elections.

“We don’t know what that could do to depress turnout,” Schriefer said.