Donald Trump famously said that he could shoot a man on Fifth Avenue in broad daylight and not lose his voters. The same is pretty much true of Bernie Sanders. His most rabid supporters will not desert him, no matter what.
The key to beating Sanders is to unite around a single realo candidate and start winning primaries in order to prevent a bandwagon effect, not to try and peel away his base. Debates don’t have that much effect on him, since he always tells the same simple story, and he hasn’t changed his positions very much over time. That said, one might make some progress with undecided voters by pointing out the tradeoffs in his ambitious programs that he consistently ignores. For example, he acts as if his GND proposal will only damage the interests of a handful of billionaire investors in fossil fuel industries. What about the hundreds of thousands–perhaps millions–of people who have good jobs that depend on fossil fuel production? Are they really going to vote themselves out of a job, and trust Sanders to provide jobs with similar pay in clean energy industries? Would you really want to take that chance if you were in that position?
Of course not. The climate issue is so compelling, it may well be necessary to swallow some significant short term pain in order to accomplish the long term objective. It is important, however, to acknowledge and justify the tradeoff–not to completely disregard it. Rest assured that the GOP will bring it up during the general election campaign if the realos don’t.