WASHINGTON (AP) -- If Donald Trump or Hillary Clinton scores a high note or commits a blunder in Sunday's presidential debate, millions of voters can respond almost immediately. They can fill out a mail-in ballot right away or head to a polling location the next day.
Early voting is on the rise in America. More than 45 million people are expected to vote before Election Day, Nov. 8, either by mail-in ballots or going to early-voting stations. Advance voting is underway in nearly half the 50 states, with more to follow. At least 403,000 people have voted already, according to data compiled by The Associated Press.
The Clinton campaign is pinning much of its strategy on the early vote, hoping to lock in less-reliable voters. Campaign manager Robby Mook pointed this past week to North Carolina, Florida and Nevada as states where the campaign hopes to build an "insurmountable lead" before Nov. 8 on the way to the 270 electoral votes needed to win.
Early voting may offer some early clues about the race. There are encouraging signs for Clinton in North Carolina, and judging by ballot requests, the first presidential debate seemed to help Democrats more than Republicans in pockets of the U.S.
But Clinton's push for mail-in voting in Florida could be hurt by Hurricane Matthew, which struck just as ballots were being mailed to voters.
The Trump campaign isn't copying the Democrats' expensive, labor-intensive efforts to promote early voting. Instead, it's leaning heavily on the Republican National Committee for its get-out-the-vote-effort and counting on mass rallies and big television audiences to spur early voting.
Things to know about advance voting trends to date:
WHY IS EARLY VOTING ON THE RISE?
Voting ahead of Election Day is designed to make it easier for people to participate in the democratic process. Absentee voting long has been a fixture in elections, and an increasing number of states have approved changes in election laws to permit early voting.
For 2016, 37 states now permit advance voting by mail or in person with little restriction; that's up from 34 in 2012.
The Clinton campaign is also seeking to stir up interest, scheduling events around voter registration and early voting dates and making calls and knocking on doors.
Preliminary data compiled by the AP suggest that advance voting could reach 40 percent of all votes nationally — up from 35 percent in 2012. The data may also indicate a higher overall turnout in an election that has generated enormous public interest.
HOW MANY VOTERS CAN CAST BALLOTS?
More than 5.6 million people who requested mail-in ballots are expected to have them on hand in the coming week, according to figures reported by the states. Florida, which began vote by mail Tuesday, had the biggest volume of ballots sent and not yet cast at 2.6 million, or nearly one-third of the state's voters in 2012.
Other states reporting tens of thousands to hundreds of thousands ballots sent or requested so far include Michigan, Iowa, North Carolina, and Virginia.
In addition, several states now allow early in-person voting, including Iowa and Illinois. On Monday, California will start mail and in-person voting. In 2012, more than 6.7 million Californians ultimately opted to cast ballots early, representing more than half of the state's total voters.
Ohio, which reported a record pace of 957,000 requests so far, mails out ballots Wednesday. It also will begin in-person voting.
COULD A PRESIDENTIAL DEBATE TIP EARLY VOTING?
Nudge it, perhaps.
Barring a major disaster by a candidate, the effect is usually more of a blip — a shuffling of likely supporters who often decide to move up their decision or wait longer to commit.
That's because independents and undecided voters are still more likely to vote on Election Day or just before.
In Iowa, for instance, requests for ballots from Republicans fell below their 2012 levels for the first time days after the first debate. Still, due to overall soft support among Democrats, Republicans aren't as far behind as they were at a similar point in 2012, when Barack Obama eventually won the state by 5 percentage points.
In Maine's rural 2nd Congressional District, Democrats previously were running behind 2012 levels but made up ground, also in recent days. Trump is counting on that district's one electoral vote to help reach 270. Maine allocates its electoral votes by congressional district.
"I suspect we're seeing a post-debate 'enthusiasm' effect for Clinton, or at least a post-debate dropoff for Trump," said Michael McDonald, a University of Florida expert who runs the U.S. Elections Project.
ELECTION DAY STILL MATTERS
Advance voting has taken on outsized importance because of Trump's narrow path to 270 electoral votes and his need to win critical states such as North Carolina, in which early voters make up more than 60 percent of all ballots cast.
In North Carolina, where Obama lost by just 92,000 votes in 2012, initial measures show Democrats running ahead in submitting ballots compared with a similar 2012 period. That is good news for Clinton, but she still needs the kind of enthusiastic turnout that Obama generated in 2012 among nonwhites and young people in the remaining weeks.
If Republicans are able to keep it close, Election Day will tip the outcome.
Trump also is seeking to be competitive in Democratic-leaning states including Pennsylvania and New Hampshire, which have low rates of advance voting. That makes it harder to gauge vote trends in those states, potentially upending the electoral map if Trump can pull off a few surprise wins on Election Day.
Colorado, Florida, Iowa and North Carolina, in which early voters make up 45 percent or more of total ballots, all report the party affiliation of people who cast ballots early, offering solid clues.
But no early votes will be actually counted until Nov. 8.