Data from the United States Election Project shows that there really wasn't a consistent link between changes in early voting percentages (horizontal axis) and changes in overall turnout for the highest contest on the ballot (vertical axis) between the 2010 and 2014 midterms or the 2008 and 2012 presidential elections.
But there are expediency reasons for Clinton to champion the cause.
Foremost among them is that early voting makes it easier for working people to cast a ballot (whether or not they choose to do so). If you work long hours on Tuesday, there will be some day during the three weeks prior that you have time to go in and cast a ballot. (The Post's Emily Badger thoroughly detailed the economic issues last year.) For a candidate who wants to appeal to working class voters, it's a ready-made issue.
The expansion of early voting overlaps with another issue that Clinton has championed: Voting access for black voters. When Ohio rolled back early voting shortly before the 2012 election, a study found that the move disproportionately affected black voters. In part, that's because black churches often run "souls to the polls" programs, in which parishioners go from church on Sunday to cast a ballot. Not a coincidence: Clinton's speech on Thursday is at Texas Southern University, an historically black school.
But there's another good political reason for Clinton to back early voting: It will likely play much more to her campaign's strengths than her opponent's.
Democrats have historically been better at turning people out to the polls. Field programs -- the process of calling voters or knocking on doors -- have been a core component of labor unions' political activity for decades. In part because Republican voters vote more regularly, Democrats have designed systems aimed at bolstering their bottom line. And while the 2013 study found that overall turnout dropped, there's an opportunity for a well-staffed, well-funded campaign to take advantage of an extended voting window. Like, say, a presidential campaign. If turnout overall drops but the number of Democrats goes up, the Democrats are not going to complain.
In 2012, exit polling suggested that Barack Obama and Mitt Romney tied among voters that cast a ballot on Election Day, but Obama won early voters by six points. If Clinton can duplicate that spread and increase the number of early voters (the number of which are already growing), she can assure a substantial lead well in advance of Election Day.
Other considerations aside, Clinton's team would find that appealing.