AUGUSTA, Maine — If voting trends from the past 12 years hold form, between a quarter and a third of Mainers who cast ballots for president this year won’t go near the polls on Election Day.

Instead, they will have already submitted their votes by absentee ballot, a circumstance that has altered the pace and focus of this year’s campaign for an open U.S. Senate seat.

The likelihood that as much as a third of the electorate will cast ballots weeks before Election Day is “changing the way campaigns operate and gives advantage to party organizations that can bank their votes early,” according to Amy Fried, a University of Maine political science professor whose Pollways blog is hosted by the BDN.

Absentee ballot applications are now available and will be through Nov. 1. After that, special hardship circumstances must apply to vote by absentee ballot. Completed ballots must be submitted by 8 p.m. Nov. 6.

With the latest polls showing independent U.S. Senate candidate Angus King’s lead narrowing over Republican Charlie Summers and Democrat Cynthia Dill, political parties’ ability to muster early votes forces the candidates to adapt their campaign strategies.

“Political campaigns are now faced with two electorates — those who vote early and those who vote on Election Day — and they must adjust the timing and sequence of campaign activities as well as tactics accordingly,” Stefan D. Haag, an adjunct professor at Austin Community College, wrote in an analysis of early voting in Texas.

In 2000, Maine switched to “no excuse” absentee balloting, which removed requirements that voters provide a reason when asking to vote as early as three months in advance by absentee ballot. Since then, the practice of casting absentee ballots before Election Day — which differs from “early voting as defined by the secretary of state’s office — has increased markedly.

In presidential elections prior to 2000, absentee balloting represented less than 10 percent of the overall vote count, according to Megan Sanborn of the Maine secretary of state’s office.

The number of absentee ballots issued in 2000 (76,672) more than doubled for the 2004 presidential election (166,226). In 2008, the number of absentee ballots issued statewide soared to 243,992.

Of all Maine votes cast in the presidential elections since 2000, the first year of “no excuse” absentee balloting, the percentage of absentee votes rose from 12 percent in 2000, to 22 percent in 2004, to 32 percent in 2008, according to the secretary of state’s office.

Nationwide, roughly 20 percent of all votes cast in 2004 and 30 percent of those submitted for president in 2008 came via absentee ballot, according to “The Effects of Early Voting on Campaign Advertising,” a 2012 study by Johanna Dunaway and Robert M. Stein.

The practice hasn’t significantly altered voter turnout in Maine, which has hovered in the range of 65 percent to slightly above 70 percent dating back to elections prior to the implementation of “no contest” absentee ballot voting. But it has affected campaign planning.

“If early voting status does in fact mean that campaigns move up the timing of campaign activities, early voting may increase the volume of issue-oriented advertising,” Dunaway and Stein wrote. “‘Get out the vote’ efforts directed at more partisan and ideological voters may require issue-oriented and attack ads to motivate the faithful to turn out.”

That supposition appears to be partially true in Maine. While political issues ads have been rare this campaign season, broadcast and online attack ads — directed at King and, to a much lesser extent, Dill, by national organizations that support Summers — have appeared with increasing regularity.

Even if those ads fail to sway undecided voters, they could play a role in marshaling party loyalists to take part in “get-out-the-vote” efforts designed to swing a tight election, especially in what’s perceived to be a three-way contest that also includes three lesser candidates on the ballot who could siphon off just enough votes to affect the outcome, Fried said.

“You need individuals who will talk to people and sign up voters,” Fried said. The state Democratic and Republican parties have long-established structures in place, which theoretically gives Dill and Summers an advantage over King, she said.

Crystal Canney, communications director for King’s campaign, recognizes that advantage. In an email, she pointed to a possible conflict of interest related to Summers’ role as secretary of state.

“The secretary of state who is running as a U.S. Senate candidate has been very aggressive about absentee balloting this year using the taxpayer-funded state website to direct people to vote early and ensuring that his name is in the public,” Canney wrote. “It might be more appropriate to have any emails or mailings come from someone else within the secretary of state’s office until the election is over. It certainly would avoid any perceived conflict of interest. Our campaign is working with our voters to get them to vote early.”

Eliot Cutler, an independent candidate who narrowly lost to Republican Paul LePage in a three-way gubernatorial race in 2010 and who supports King’s U.S. Senate run, believes that, rather than focusing on attracting early voters, King needs to continue to frame the differences between himself and candidates representing the Democratic and Republican parties, which he described as “vanishing species.”

Cutler decries early “convenience” voting as “anti-democratic” and calls it an example of Maine electoral processes — including the lack of open primaries and runoff elections — that are “rigged to benefit political parties.”

“I am hard pressed to come up with a case to support voting six weeks before an election,” he said. “Convenience voting is an invention of the political parties who can use it to drive voters to vote before they know what their choices are. The electorate ought to be given an opportunity to get to know candidates — who they are and what they stand for — before they make a choice.”

If King remains the front-runner, early voting could work in his favor as an obstacle to a trailing candidate who makes a late surge. The former governor doesn’t suffer from the same lack of name recognition that placed Cutler in a position to have to come from behind the major party candidates.

As Cutler’s 2010 gubernatorial bid gained momentum and Democrat Libby Mitchell’s floundered, the Cutler campaign suggested that voters who had submitted early ballots for Mitchell change their votes. A spokeswoman for the secretary of state’s office said requesting a new ballot simply because a voter wanted to change his or her candidate choice would conflict with state law.

“If a voter submits an absentee ballot and it is accepted by the municipal clerk, that voter is not allowed to vote again — either by requesting an absentee ballot or by voting at the polls,” Sanborn, who works for Summers in the secretary of state’s office, wrote in an email. “ The Legislature passed a law stating that once a voter has submitted her or his absentee ballot, the voter cannot be issued another ballot.”

A similar scenario to Cutler’s late rally in 2010 might compel some voters to delay casting ballots if they’re motivated to ensure that one party or candidate doesn’t eke out a victory, Fried said.

“I think there’s still a hangover from 2010 when Democrats voted early, then wanted to change their vote [when they perceived that Cutler had a better chance than Mitchell to beat LePage],” Fried said. “It could prevent Democrats and Democrat-leaning independents from voting for Dill now.”

“I would hope so, frankly,” said Cutler.

Robert Long is a political analyst for the BDN.