From Nate Cohn’s “Early Voting Numbers Look Good for Democrats” at The NYT Upshot:

Democratic efforts to turn out the young and nonwhite voters who sat out the 2010 midterm elections appear to be paying off in several Senate battleground states.
More than 20 percent of the nearly three million votes already tabulated in Georgia, North Carolina, Colorado and Iowa have come from people who did not vote in the last midterm election, according to an analysis of early-voting data by The Upshot.
These voters who did not participate in 2010 are far more diverse and Democratic than the voters from four years ago. On average across these states, 39 percent are registered Democrats and 30 percent are registered Republicans. By comparison, registered Republicans outnumbered Democrats in these states by an average of 1 percentage point in 2010.
The turnout among black voters is particularly encouraging for Democrats, who need strong black turnout to compete in racially polarized states like Georgia and North Carolina. In those two states, black voters so far represent 30 percent of the voters who did not participate in 2010. By comparison, 24 percent of all those who voted in those states in 2010 were black.

All of that said, The Upshot’s turnout model still gives the Republicans a solid edge in the battle for a Senate majority, owing to their historically superior midterm turnout patterns. Nonetheless, adds Cohn, “…the figures are still good news for Democrats. The early-voting surge gives them a chance to pull off upsets in crucial states, particularly if they continue coaxing new voters to the polls in the final week of the campaign.” Read Cohn’s entire post for more data analysis.