From William Galston’s Wall St. Journal column: “What It Will Take to Win in 2016
Democrats have some advantages, but history favors the Republicans“:

Some basics favor the Democrats. Relative to 2012, the share of the electorate commanded by minority voters–principally Hispanics, African-Americans and Asians–will continue to increase. This shift will be especially pronounced in states such as Florida and Nevada, where large numbers of Hispanics are reaching voting age. Millennial voters (young adults of all hues) will also increase as a share of the electorate, and they tend to favor Democrats as well.
In these highly polarized times, party identification does more to shape voting behavior than it did decades ago, and recent studies show that the Democrats have retained their advantage. To be sure, as the share of the electorate that regards itself as independent surges, both parties continue to decline. But most self-professed independents lean toward one party or the other and vote accordingly, and true independents constitute at most 15% of the electorate. The most recent NBC/Wall Street Journal survey gave the Democrats a six-point edge (42% to 36%) once leaners are taken into account.

But Galston also notes that “the Democrats’ advantage in the vital Midwest–home to six of the 11 states decided by single digits in 2012–has disappeared.” Further, “Emory University political scientist Alan Abramowitz has shown that candidates vying to succeed incumbent two-term presidents of their own party face an uphill climb–all else being equal, a penalty of between four and five percentage points relative to the incumbent’s second-term share of the vote.”
Galston also warns that President Obama’s ‘approval drag’ effect is a significant problem: “On average, a five-point increase in public approval will increase his would-be successor’s share of the popular vote by 0.9%. Mr. Obama’s rating, which topped 50% in the concluding days of his re-election campaign, is now hovering around 45%–a modest but potentially significant drag on the Democratic nominee’s prospects if not reversed.
But Democrats have reason to hope that the economy will break their way in the months ahead, according to CBO growth projections of 3 percent for 2016, especially if it produces an employment uptick. A woman nominee and disgust with Republican voter suppression and immigrant-bashing could give Democrats the edge they need with key constituencies, adds Galston.
“…If heads ultimately dominate hearts in both parties,” concludes Galston, “the 2016 election will be closely contested. However, “if the GOP selects a nominee who can appeal to Midwestern voters, the Democrats’ “lock” on the Electoral College could go the way of its Republican predecessor in 1992.”
Galston’s cautiously optimistic assessment for Democrats makes sense. But it is tempered by a significant concern about growing Democratic weakness in the midwestern states with respect to the electoral college. An extra effort on the part of the national and state Democratic parties in those states, along with well-targeted contributions to GOTV in the midwest, just might make a pivotal difference in 2016.