History will undoubtedly record the odyssey that has been Bernie Sanders' improbable quest for this year's Democratic presidential nomination.

On the day he launched his campaign, in May 2015, the left-wing firebrand trailed Hillary Clinton by more than 50 points. One year later, he has won over 11 million votes, amassed victories in more than 20 states and fought the former First Lady to the finish line.

But it is the next six weeks that will determine history's final verdict. As voters went to the polls in the final major primaries this week, Mrs Clinton passed the 2,383 delegates she needed to become the party's standard-bearer in November. She capped her achievement with wins in California, New Jersey, New Mexico and South Dakota, while her rival took Montana and North Dakota.

Even as she did so, Mr Sanders' vowed to fight on to the Democrat convention next month in Philadelphia. His only hope now of wresting the nomination from her, though, is to persuade the party's "superdelegates" - officeholders who are not bound to support a particular candidate in the vote to pick the nominee - to abandon their hitherto solid support for her. Given that Mrs Clinton leads him by more than three million in the popular vote, it is a curious tactic for a man keen to present himself as a political outsider. Mr Sanders will rest his case on polls that appear to show him better able to defeat Donald Trump.

Mr Sanders will now come under intense pressure to follow Mrs Clinton's example when she lost the primary battle to Barack Obama eight years ago and bow out gracefully.

Already, prominent liberals are rallying behind the former Secretary of State and party leaders who thus far have remained neutral, such as the Democrats' leader in the House of Representatives, Nancy Pelosi, have endorsed her. Mr Obama himself is expected to do so shortly. That pressure is heightened by the fact that with, with Mrs Clinton and Trump neck-and-neck in the polls, many Democrats fear that Mr Sanders' continued presence in the race is preventing her from unifying the party.

In particular, they worry that his harsh anti-establishment tone and failure to shut down the notion prevalent among his supporters that the race has been "rigged" against their candidate could do Mrs Clinton lasting damage. Polls indicate as many as one in four of Mr Sanders' backers say they will not vote for Mrs Clinton in the general election.

Mr Sanders will be the author of the final chapter in this story. By rallying his supporters behind Mrs Clinton and helping her to victory in November, he will be remembered as the most successful Jewish presidential candidate ever. But if he fights on, splitting the party and allowing Mr Trump into the White House, he will instead face inevitable comparisons with Ralph Nader, the left-wing candidate whose dogged presence in the presidential race 16 years ago narrowly handed victory to George W Bush over Al Gore.