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Updated question

Is expanding the electorate a foolish strategy in American politics?

One part of Bernie Sanders' strategy to become the Democratic party's presidential candidate in 2020 was to appeal to groups that generally vote less than other groups. E.g. young people, low income and minority groups. In an interview Democrat-aligned political strategist James Carville compared the strategy to climate change denial (i.e. something conclusively proven wrong by science):

"The entire theory" put forward by Bernie Sanders "that by expanding the electorate, increasing turnout so you can win an election, is similar to climate denial," Carville said. "When people say that, they're as stupid to a political scientist as a climate denier is to an atmospheric scientist."

"If you want to vote for Bernie Sanders because you feel good about his program, you don't like the banks on Wall Street or you don't like pharmaceuticals, that's legitimate, I understand that. If you're voting for him because you think he'll win the election, politically, you're a fool."

James Carville: "You're A Fool" If You Think Bernie Sanders Can Win By Expanding The Electorate

Carville claimed that there is "no debate about this" and that this part of the Sanders campaign's strategy was completely wrong.

What evidence, if any, was Carville relying on? Is the evidence so strong that anyone who disagrees with it can be compared to a climate change denier?

Since the context of Carville's statements was American politics, I'm only interested in sources about American politics. I'll also underscore that the question is not about James Carville or Bernie Sanders - the question is whether strong evidence supports the claim that "expanding the electorate" is an unworkable strategy in American politics.