For millions of Americans, Election Day is already over. Thirty states now allow no-excuse absentee voting, and most of them also allow...

WASHINGTON — For millions of Americans, Election Day is already over.

Thirty states now allow no-excuse absentee voting, and most of them also allow voters to cast early ballots in person at county clerks’ offices or satellite polling places.

In Montana, absentee ballots were mailed Sept. 22. Oregon’s elections are conducted entirely by mail, and Washington is moving in that direction. California sent out 3.8 million absentee ballots the week of Oct. 8.

Candidates are maneuvering to adapt to a changed political calendar, accelerating their advertising, their mailings and their get-out-the-vote calls. They are figuring out exactly who votes early and are trying to get to them before they cast their ballots. They are raising more money and spending it faster.

“Love it or hate it, it’s the wave of the future,” said Art Torres, chairman of the California Democratic Party. “Election Day started here on Oct. 10 and lasts 29 days. It’s tremendously burdensome on our fundraising and the people we have out in the field.”

Experts estimate that more than 20 percent of voters nationwide will cast their ballots before Election Day by mail or at early-voting locations, a proportion of the electorate that is rising with each election. Some states and counties open the ballots before Election Day and keep the results secret; others count them with regular ballots.

Analysts and party officials who study early voting trends say that a decade ago those who took advantage of absentee ballots tended to be relatively well off and highly educated, with Republicans outnumbering Democrats by almost 2-to-1. But as the ease of early voting has spread, the ratio is slipping, and some analysts say that nearly as many Democrats as Republicans now vote early.

Those who favor the practice say it is convenient for voters and increases turnout. Most elections officials welcome the trend because it reduces the strain on polling places and poll workers on Election Day.

But some experts say there is no proof that early voting increases turnout, and it may well have the opposite effect because some voters request absentee ballots and then neglect to send them in. They are also concerned that absentee ballots are more open to fraud than those at polling places.