ATLANTA — Midterm elections are almost three weeks away, yet more than 904,000 Americans already have cast their ballots, with almost 60 percent of those early votes in Florida, according to data compiled by The Associated Press from election officials in 11 states.

Those numbers are climbing daily as more states begin their advance voting periods and more voters return mail-in ballots ahead of Nov. 4.

Early voting doesn’t predict electoral outcomes, but both major parties emphasized the opportunity in recent elections as they try to lock in core supporters. Thirty-three states and the District of Columbia allow some form of advance voting other than traditional absentee voting requiring an excuse.

A spokesman for national Democrats, Justin Barasky, said the party is especially focused on encouraging early voting by Democrats who usually don’t participate in midterms. Returns show that, historically, casual voters who support Democrats in presidential elections are more likely to stay home than average Republican voters.

In the 2010 midterms, when Republicans regained control of the House and won sweeping victories in statehouses around the country, advance voting accounted for almost 27 million ballots out of more than 89 million, meaning about 3 out of 10 voters cast early ballots. Almost 129 million people voted in the 2012 presidential election, 35.8 percent of them before Election Day.