More than 6 million Americans have already voted in 27 states for November’s general election, according to data released by states that have begun accepting ballots.

Registered Democrats have returned 1.4 million ballots, more than twice the 653,000 ballots registered Republicans have returned so far, according to Michael McDonald, a political scientist at the University of Florida who analyzes early voting.

About two-thirds of voters who have already voted — 3.7 million Americans — are either unaffiliated with either party or live in states that do not register voters by party. Demographic modeling by one prominent Democratic firm, TargetSmart, estimates that almost 3 million of all votes cast have come from Democratic voters, compared to about 2.1 million from Republicans.