Let me say it loud and clear: Hillary Clinton is his election’s Woodrow Wilson at this moment. A candidate who personally is not as progressive as you would like to think, but would literally listen to a progressive movement in order to keep her party happy. But, if Bernie Sanders were to either run as an Independent (which he won’t), be written in by his #BernieOrBust fans, or let his voters refuse to vote for a Clinton campaign — the election would be swapped. Clinton would no longer be the victorious Wilson, she would be left in the dust with a low turn out rate or split vote as the 2016 equivalent to William Howard Taft.
I will spoil this for you: your third party candidate cannot and will not win in a political bipartisan society dominated by DEMOCRAT and REPUBLICAN. Your write-in of Bernie Sanders will not win against an official DEMOCRATIC nominee with an electoral vote dependent on getting near the White House. A vote for anyone against the Democratic nominee — or an absence of a vote for said Democratic nominee — will lead to four years of Donald J. Trump.