Put aside all the daily campaign controversies, and the 2016 election will hinge on one thing: Will the coalition of voters who twice showed up in large numbers to elect President Obama turn out to vote for Hillary Clinton, the assumed Democratic nominee?

Horse race polls, favorability ratings, those will all fluctuate many times between now and next November. But if Clinton can't find a way to engage unmarried women, millennials and minority groups — it's going to be over for her.

That's why this Democratic poll released on Monday, taken by Greenberg Quinlan Rosner Research on behalf of several liberal groups, should be seen as so troubling for Democrats.

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Though, on the surface, it points to a real opportunity for Democrats to retake control of the Senate, beneath the surface it shows a lack of interest among millennials, unmarried women and minorities relative to the interest being shown from more traditional Republican voting blocs.

Specifically, 71 percent of seniors said they were "extremely interested" in the presidential election; 70 percent of conservatives and 70 percent of Republicans said the same. In contrast, just 65 percent of Democrats, 63 percent of minorities, 60 percent of unmarried women and 42 percent of millenials said they were "extremely interested."

Liberal Greg Sargent of the Washington Post notes that, "The basic problem outlined by Greenberg ( and noted by other Dem pollsters) is that, even if Democratic economic policies are broadly popular, this isn't enough on its own, because many Americans don't believe government can or will actually deliver on those policies."

This is a key point, and the challenge for Clinton is two-fold. First, she isn't as inspiring of a figure as Obama. And secondly, after nearly two-terms of Obama, who saw his promises of a wave of change come crashing against the realities of Washington, there's more disillusionment among voting groups who have been attracted to the liberal economic message in the recent past.