Ex-New Republic journalist John Judis has a piece on Talking Points Memo today in which he explains that his intention to vote for Bernie Sanders in the Maryland primary on April 26—he has even planted a Sanders lawn sign in front of his house, pictured in the article—should not be taken as evidence that he actually wants Sanders to be the Democratic nominee. In fact, says Judis, his and his wife's support for Sanders is "on the condition that we don't think he will get the nomination."
For Judis, supporting Sanders is a strategic act—to express approval for some of the issues that the Sanders campaign has raised, to push Hillary Clinton further to the ideological left, to lay the groundwork for a more ambitious national Democratic policy agenda in the future. But if Sanders were actually nominated in 2016, Judis worries that his favored candidate would have a difficult time winning a general election, and he is also "not sure whether [Sanders] is really ready for the job of president."
My anecdotal experience suggests that Judis is not alone. Sanders is winning some votes from Democrats who don't necessarily think that he would be the best nominee for the party, or even the most effective president, but are supporting him in the primaries because they wish to express their endorsement of his views and/or send a message to Hillary Clinton to pay more attention to the party's left wing.
It's hard to know exactly how large the contingent of "Vote Bernie, Hope Hillary" voters is, and doubtless they are outnumbered many times over by Sanders supporters who sincerely prefer him to be the Democratic nominee. But this phenomenon might account for part of the seesaw pattern of primary outcomes that we've seen this year on the Democratic side: a Clinton victory in Iowa was followed by a Sanders landslide in New Hampshire, which in turn was followed by Clinton wins in Nevada, South Carolina, and most of the Super Tuesday states—which then led to a Sanders upset in Michigan before Clinton swept on March 15, after which Sanders won 7 of 8 states leading into next week's New York primary.
As long as Clinton appears to have a lock on the nomination, Judis-style voters are free to indulge their symbolic support for Sanders. But if Clinton seems even slightly vulnerable—as after well-publicized Sanders victories—these voters might be less likely to depart from their sincere preference that she win the nomination, instead rallying behind her in the next round of voting. Of course, much of the apparent momentum shifts in the Democratic contest reflect differences from state to state in demographics and electoral rules, not changes in the actual dynamics of the race. But there may be an element of voter strategy at work here too.