WASHINGTON — Who's that at the doorbell this sleepy August? It could be supporters of Hillary Clinton and Donald Trump, eager to pounce well before what used to be the traditional Labor Day start of a general election campaign.

Thirty-seven states, including the swing states of Florida, North Carolina and Ohio, now have some sort of pre-Election Day voting, and that means no hot-weather letup in the push for votes.

What it most means is "hand-to-hand combat," said Scott Jennings, a political veteran who ran Republican nominee Mitt Romney's 2012 Ohio campaign.

The campaigns know how to find you. Republicans "score" voters and know who's most likely to support or oppose them. Clinton's campaign has been working since April to get people out to vote. Volunteers are standing by, ready to address any concern.

"It certainly changes the way people campaign," said Colorado Secretary of State Wayne Williams, whose office oversees that state's elections. Colorado mails ballots to every eligible voter, and opens early polling places in October.

While anecdotal evidence shows that in many states, Democrats stand most to benefit from early voting, Bob Hall, executive director of Democracy North Carolina, a nonpartisan voting rights advocacy group, said the pre-election-day voters tend to be those most enthusiastic about a candidate. In North Carolina, that meant more African-Americans were likely to vote early in 2008 and 2012, when President Barack Obama ran. But in recent off-year elections, it was the white Republican voter who were more drawn to cast ballots early.

The Obama campaigns were so effective at identifying early voters, that in 2008, "we knew going into Election Day we had won" in Florida, said Steve Schale, Obama's state director that year. Among their strategies: Having surrogates lead marches and rallies to early polling spots.

This year, it's tough to tell who will get the early voters. Both Clinton and Trump have solid cores of support, polls find, but their negatives remain at historically high levels.

The first debate, scheduled for Sept. 26, is crucial to the early voting push. "By the time of the third debate, the percentage who was persuadable was way down," Jennings said.

For the campaigns, that means planting in viewers' minds impressions of both their candidate and opponent so they'll be strongly reinforced in that debate.

Republicans are pounding away at the idea that Clinton is dishonest. Earlier this week, a federal judge told the State Department to release emails earlier than the originally planned timetable in mid-October.

Republicans heartily agreed. "The process for reviewing these emails needs to be expedited, public disclosure should begin before early voting starts, and the emails in question should be released in full before Election Day," said Republican Chairman Reince Priebus this week.

Republicans in North Carolina, though, are encouraging GOP members of county election boards to "make party-line changes to early voting" that include limiting the number of hours and having polling places stay closed on Sundays, the News & Observer reported. It found through a public records request that the party's state executive director emailed the request to Republican county board members and other party members Sunday.

Clinton is fighting back by raising her own doubts right now about Trump. Last week, he made efforts to woo African-American voters, who have been solidly with Clinton. This week, Marlon Marshall, the Clinton campaign's director of state campaigns, accused Trump of "bigotry."

Backing it up: volunteers going door-to-door, now or very soon, reminding voters of all these points.

The Republican Party identifies voters nationwide with scores ranging from zero to 100. Among the items rated are whether the voter is likely to support a Republican and what issues matter most.

Republican volunteers from each neighborhood are dispatched to get in touch with the potential early voters. Starting in June, they visited their homes or made phone calls. If controversies about Trump arose, the GOP volunteers were ready to discuss.

They pay particular attention to trends in the scores over time — the scores are updated every two weeks. Volunteers will return, usually in the evening, when party officials have determined is the best time to reach voters. So far this election cycle, they've already knocked on at least 244,594 doors in North Carolina and 403,726 in Florida.

Among Democrats, Clinton's campaign has been working in swing states since the late April primaries, devising a plan to take advantage of all the methods a voter could use to vote.

They're using their research to identify early voters, then contacting them to urge signing up for absentee ballots or be aware of early voting procedures.

In states such as North Carolina, Florida, Nevada and Colorado, the Clinton voter protection team is working with county officials and local governments to be sure there are early voting locations. Next, they contact voters to tell them where those locations are.

In Florida, many counties will automatically send an absentee ballot to people who voted that way four years ago. But in counties that don't do that, or in cases where people didn't vote in 2014, the campaign is contacting people with a reminder to request those ballots.

Clinton herself pushes the efforts at her rallies. "If you aren't registered and you're eligible, see the persons with the clipboards here," she told a rally in Orlando.

Buttressing the ground game are sweeping ad campaigns in the too-close-to-call states.

Clinton plans to spend an estimated $2.1 million in Florida between Aug. 19 and 29, while Trump spends $1.8 million. Each plans to spend around $950,000 each in North Carolina during that period, while Clinton spends $1.1 million in Ohio and Trump spends $819,000. The data were compiled by Advertising Analytics, which tracks such information, and NBC News.

Getting one's backers to vote at least has a huge tactical advantage. Once they get the candidates' biggest backers to the polls, "their name goes off the list of people getting phone calls," said Williams. That way, the campaigns can spend the final days wooing the swing voters.