In his New York Magazine post, “Why Hillary Clinton Is Probably Going to Win the 2016 Election,” Jonathan Chait shares a six-pack of reasons for the argument encapsulated in his title. He leads with his strongest point:

1. The Emerging Democratic Majority is real. The major disagreement over whether there is an “Emerging Democratic Majority” — the thesis that argues that Democrats have built a presidential majority that could only be defeated under unfavorable conditions — centers on an interpretive disagreement over the 2014 elections. Proponents of this theory dismiss the midterm elections as a problem of districting and turnout; Democrats have trouble rousing their disproportionately young, poor supporters to the polls in a non-presidential year, and the tilted House and Senate map further compounded the GOP advantage.
Skeptics of the theory instead believe that the 2014 midterms were, as Judis put it, “not an isolated event but rather the latest manifestation of a resurgent Republican coalition.” Voters, they argue, are moving toward the Republican Party, and may continue to do so even during the next presidential election.
It has been difficult to mediate between the two theories, since the outcome at the polls supports the theory of both the proponents and the skeptics of the Emerging Democratic Majority theory equally well.
A Pew survey released this week gives us the best answer. Pew is the gold standard of political polling, using massive surveys, with high numbers of respondents and very low margins of error. Pew’s survey shows pretty clearly that there was not a major change in public opinion from the time of Obama’s reelection through the 2014 midterms:

Of course, Pew is not surveying actual voters. It’s surveying all adults. But that is the point. What changed between 2012 and 2014 was not public opinion, but who showed up to vote.

Chait goes on to acknowledge that “The Emerging Democratic Majority thesis places a lot of weight on cohort replacement.” But he also notes that “every new election cycle incrementally tilts the electoral playing field toward the Democrats,” despite the oft-cited Harvard Institute Poll of millennial voters (which had flunked its prediction that millennial voters would favor the GOP in 2014). Further, younger millennial voters are still tilting pro-Democratic in the Pew poll.
Chait’s other points are well-rooted in reality as well. And it’s not like the Republicans have yet left the fever swamps to adopt a more temperate approach to win moderates. Indeed, as Chait puts it:

The argument for Clinton in 2016 is that she is the candidate of the only major American political party not run by lunatics. There is only one choice for voters who want a president who accepts climate science and rejects voodoo economics, and whose domestic platform would not engineer the largest upward redistribution of resources in American history. Even if the relatively sober Jeb Bush wins the nomination, he will have to accommodate himself to his party’s barking-mad consensus. She is non-crazy America’s choice by default.

It’s good to have political demographics on your side, and Chait is surely right that sanity is a big plus as well.