Democrats are currently panicking over whether Hillary Clinton will be able to reach out and appeal to Bernie Sanders's supporters in the general election or whether Sanders will be "willing to harm" Clinton's prospects in order to advance his own agenda.
History suggests there's probably no need to worry about this. Candidates always fight hard until they stop fighting — look at what Clinton herself was doing in mid-May 2008 — and when Sanders does stop fighting he'll have strong incentives to do everything possible to get Democrats to vote and capture the Senate.
The real problem for Clinton is that capturing the votes of people who bother to become passionately invested in a presidential primary campaign is actually an incredibly low bar. She probably can't get the vote of literally every single person who cast a ballot for Sanders this primary season, and even if she could, that wouldn't be nearly enough people. To win, she needs to mobilize not just Sanders's supporters but the much larger universe of people with left-of-center views who didn't vote in the primaries at all.
Nobody votes in primaries
Consider this. Back in 2008, Barack Obama got 17,584,692 votes, and Clinton lost despite winning about 260,000 more votes than he did. It was a hard-fought campaign, and people worried about party unity.
But come November, Obama didn't just get the 35 million or so votes that he and Clinton combined for in the primaries. He got 69.5 million votes — equivalent to about double the total number of people who voted in the primary.
The Clinton-Sanders race isn't nearly that close, but it's also been an even lower-turnout affair. Sanders has only about 10 million votes, and Clinton has about 13 million. Both of those numbers will go up once California and New Jersey vote, but they're not going to equal the levels we saw in 2008.
General election winners need way more votes than the combined Clinton-Sanders total. Obama got 66 million votes in 2012, George W. Bush got 62 million votes in 2004, and even in the low-turnout three-way race way back in 1992, when the population was lower, Bill Clinton got 45 million votes.
Sanders voters are the low-hanging fruit
Given all that, the reality is that out of the tens of millions of people whom Clinton is going to have to win over to her side, the people who got out and voted for Sanders in the primary are some of the lowest-hanging fruit. They are, by definition, people registered to vote already. And they are people who are taking an unusually high level of interest in politics.
Their names and contact information will be on Sanders's campaign lists, and Sanders will be endorsing Clinton and campaigning for her. Some Sanders voters, naturally, will be unpersuaded, but for most of them this is going to be easy work.
The real problem is elsewhere — with the legions of people with left-of-center political views who are less attached to the political process than actual Bernie Sanders voters.
Democrats are fickle about voting
Rainy days hurt turnout across the board, but they are especially bad for Democrats.
By the same token, while turnout falls across the board in midterms it falls further for Democrats.
Younger people and nonwhites, in particular, seem less connected to the political process and more willing to stay home from the polls.
Obama's relatively narrow reelection in 2012 was driven by his ability to get those fickle voters to come to the polls in a way they didn't in 2010 or 2014.
And here's where Clinton has a problem. Her weakness with young voters was evident in both the 2008 and 2016 primaries. And even if she can do a good job of reaching the young people who got turned on to politics via the Sanders campaign, to win she needs to reach a lot of young people who haven't stirred themselves to vote for Sanders — which is going to be a harder challenge.
The perils of relying on negative partisanship
Democrats' current hope is that fear of Donald Trump will be an excellent motivator to drive turnout among the Obama coalition of minority groups and young people. And certainly it's hard to imagine choosing a better opponent for that purpose.
But it's far from clear that this sort of negative focus is a successful voter turnout strategy.
In general, after all, voting is not a very rational course of action. The odds that your decision to vote or stay home will actually be the difference between Donald Trump ending up in or out of the White House are incredibly small.
People vote for the same kinds of reasons that people attend political rallies — we're communal beings, and it's nice to sometimes take part in larger activities. I remember showing up to unprecedentedly long lines at my mostly black precinct in 2008, full of African-American voters who were voting not to help Obama secure DC's three electoral votes, but to participate in history.
Clinton, who would be the first woman president if elected, certainly has the potential to play that kind of role in people's thinking. But her generally low approval ratings, the fact that she's getting fewer votes than she did eight years ago, and her weak performance in the primary with young women are all signs that she's struggling in practice to do it.
The best news for Clinton is that Trump is viewed even less favorably by the public and will face his own massive challenges in securing Republican turnout. But the basic dynamics of a race to the bottom in terms of voter enthusiasm aren't all that favorable to Democrats — as we've seen in the past two midterm elections.