Sen. Bernie Sanders of Vermont has repeatedly said he will not mount a third-party campaign against Democratic front-runner Hillary Clinton if he ends up losing the party's nomination. This is in stark contrast to a candidate like Donald Trump on the Republican side, who has switched his position on whether or not he will support the eventual GOP nominee depending on how much support he has on any given day.
Sanders clearly does not want to have a Trumplike effect on the Democratic Party, particularly against a Republican candidate like Trump himself or Sen. Ted Cruz of Texas. His wife, Jane Sanders, has also rejected calls from Trump himself for Sanders to run an independent campaign, saying the Vermont senator has been "very clear from the beginning" that he would not play the role of a spoiler candidate during this election, according to CNN.
But what if the Republicans try to spoil their own nominee's chances first? As David Frum notes in a recent article for The Atlantic, Trump rose to the top of the GOP primary race by shoving aside Republican elites and marginalizing traditional GOP constituencies; his candidacy shows that the religious right has lost most of its power since President Ronald Regan was in power and has given way to "New York values," while the billionaire refuses to defer to the "economic expertise" of extreme budget-cutters like House Speaker Paul Ryan when speaking on economic issues.
This, of course, is in addition to the brash and often boorish public behavior of the candidate, which only fuels more anger from the GOP establishment. Indiana will be a crucial test: if Trump loses the primary there, then Republican elites will continue their strategy of denying him delegates and attempting to nominate a different candidate. This would almost certainly induce either a Trump third-party run, or at the very least it may severely hurt the GOP's voter turnout in the general election. But if Trump wins, then the delegate race is more or less over and then Republicans may feel induced to mount a third-party run against him, given the mutual animosity and the desire for many GOP elites to see Trump lose.
Either of these scenarios -- Trump running third-party/bowing out of the race, or the Republicans running a third-party candidate -- would open up a spot for Sanders to continue his campaign as an independent without playing the role of a "spoiler."
Why should Sanders even consider this possibility, given the fact that he has repeatedly indicated he will not run a third-party campaign?
His campaign has, for nearly a year now, attempted to steer the political conversation sharply to the left. He has undoubtedly succeeded in that, given Hillary Clinton's comparatively left-wing campaign in relation to her 2008 campaign or either of her husband's campaigns.
If Sanders ran third-party, it would indicate to the Democrats that the party has not done the work of trying to offer substantial policy concessions to a candidate who has competed vigorously with Clinton in many states. It would show the Democrats that they need to put in more work of retaining the loyalty of the voters who put them into office rather than trying to offer fig leaves to Republicans by mostly ignoring the party's left-wing, which is what President Clinton was famous for and which was part and parcel of Hillary Clinton's 2008 campaign. An independent run from Sanders would show the Clintons the writing on the wall: the era of triangulation is over.
It is unlikely he could win in such a scenario; Mrs. Clinton would probably sweep a four-person race fairly easily. But it would show that there is a political space to the left of the Democratic Party for those candidates, like Sanders, who are ambitious enough and who have the right policy ideas to fill it. And a substantial defection of the left in subsequent elections would force the Democratic Party to either change or shrink.