Can Bernie make the case that he would be more electable?

[Democratic Cynicalism]

He can try. But the vast majority of people recognize that he's untested and hasn't been subjected to any attacks, and therefore take his favorability/GE numbers with a pillar of salt.

The only time Clinton has been tested nationally is 2008 and 2016, but not in a general election. She lost in 2008. She has yet to be tested in the general election. I think either Sanders or Clinton would beat Trump, so this issue is essentially moot. Does anyone really think that someone as unpopular as Trump could beat either of them. The national polls showing Sanders doing better than Clinton because he isn't as unpopular as Clinton. She is only popular among Democrats. What is there that can be used against him in the general that hasn't already been said. People agree with him on the issues. The bogus argument that people won't vote for him because of the socialist label just won't fly.
There isn't any reason to think he won't do well in the general election. He is drawing large crowds. Millenials will come out big for him in November. They haven't come out for him in the primaries, but most people don't bother to vote in primaries. Yes, of course he will do at least as well in the general election as Clinton would, and probably much better. There is literally no reason to think otherwise. Remember, Bill Clinton couldn't even get a majority in either 1992 or 1996. Hillary would do better than that because of the alternative: Trump. But there is no reason to believe that she would do better than Sanders.

It's true that Sanders has higher favorables and better numbers in matchup polls than Clinton. It's also true that Sanders has a vastly smaller national reputation than Clinton and has been much less of a target of media attention, including that of conservative media, than Clinton. Having a relatively small national reputation and relatively little attention from conservative apparatchiks are two things he would not possess as the Democratic nominee. Would his favorables be worse than Clinton's would be? Probably not, but they would not be as good as they are now - that is certain.

"Electability" is a junk line on a resumé, anyway. If I want to vote for an ex-junkie swinger who's lost 12 elections for county clerk, I'll vote for them - it's up to the party to produce a candidate with better policies and principles.

[°Leprechaun]

He is already doing that and the superdelegates will eventually switch their allegiance to him

It is certainly reasonable for him to try to get them to switch, but he should still be aiming for 50%+1 of the pledged ones, to make his victory more likely. This may look impossible at this point in time, but he is still rising and (hypothetically, at least) could still win this, but he needs to start winning primaries like Wisconsin, (possibly New York or at least tie there) and most important PA and other 4/26 states. Clinton has a lot of baggage unlike Sanders and would certainly have a harder time in the general than Sanders. I don't think he needs to make the case that he will do better, but he should, because some people just don't get it. The spin by Clinton supporters is not surprising and I can forgive them for they know not what they do. Their case is weak on this particular issue. No doubt about it.