With the California Primary (the largest primary in the country for the Democratic party) just days away, it seems as though it could be the vital turning point for both the Sanders and Clinton campaign. If Clinton wins, she has essentially assured her nomination for presidency, especially given that she has a much larger number of super-delegates than Senator Sanders. With just a few other primaries on the same day as California and the District of Columbia the week after, winning California could prove to be a large momentum shift towards Senator Sanders, which—along with a few other, smaller state wins—could sway some superdelegates towards favoring him before the Democratic Convention. It's not too late to start being a Bernie Sanders supporter, and here's why it's imperative that you become one (especially if you're living in a state where the Democratic Primaries still haven't happened yet.)
Although I don't want to encourage strict, party-line voting in the presidential election, it's important that a Democrat gets into office, especially with Donald Trump as the Republican candidate. If Clinton were to become the Democratic Nominee in the general election, she would only hold a slim two percent advantage over Donald Trump, which could easily change for the worse over time, especially since this margin has decreased by well over 10 points in the last few months. On the other hand, Bernie Sanders has a much larger lead if he were to become the nominee, according to polls, which show him at about a 10.5 percent lead over Donald Trump, which would be a much larger lead for Trump to overcome. Furthermore, both Trump and Clinton are polling quite negatively in favorable/unfavorable polls, whereas Senator Sanders is on average around +10, while Trump and Clinton hold averages in the negatives.
If Clinton were to become president, it would be hard to judge whether or not she would implement the policy that she has been saying she will throughout her campaign. Clinton tends to take both sides of many different policy arguments, and she does this to appeal to the specific demographic that she is targeting at the time, or just the majority of the population in general. For example, Clinton opposed same-sex marriage for much of her political career, until it became dangerous to do so, hence she recently switched to being pro-same-sex marriage as the polls changed to favor such a viewpoint. Even in terms of trade agreements, Clinton has been known to change her positions on them as she supported and later opposed both the North American Free Trade Agreement (NAFTA) and the Trans-Pacific Partnership (TPP,) that she deemed beneficial. On the other hand, Senator Sanders has consistently voted against free trade agreements like these, as he was aware of the negative economic impacts of them that Clinton only seemed to discover recently.
A Clinton presidency is far from guaranteed, especially given the tight race she would have with Donald Trump for presidency. Even if she were to become president, the voters wouldn't be certain that she would implement the policies that they have caused them to vote for her. Thus, Senator Sanders provides the best chance of defeating Trump in a general election, while giving some level of certainty to his policy.