This morning's Washington Post has an article about early voting, which now exists in 32 states, including in some of the states with the most contentious contests such as Nevada.
It remains to be seen what effect this early voting will have on the midterm results, but it is clear from the Post report that the Democrats have been organizing in an effort to get their voters to the polls early in a way the Republicans have not. Historically, in a midterm election, many voters do not exercise their franchise. Many of the people who turned out for Obama in 2008 were young voters and minority voters, who are especially prone to sit out a midterm.
One other point. All the polls we are looking at have some type of model to distinguish likely voters from registered voters. Poll after poll has showed the two parties roughly tied among registered voters but, because of an "enthusiasm gap," the GOP opens up a lead among likely voters. The Democrats ground game, trying to get unlikely voters to the polls early, is their best, indeed, their only shot at retaining Senate seats in Colorado, Washington state and Nevada. We will have to wait and see if it works.