Bernie Sanders and an independent Kasich, who would lead to a reelected Trump
No Democrat would vote Kasich over Bernie.
Yes, yes they would. Many moderate democratic voters would be more in league with Kasich than Sanders. Believe it or not, some deomocrats aren't socialists and would have real trouble voting for a socialist.
But they'd take Bernie Sanders over risking 4 more years of Donald Trump.
Just like how moderate Mormon republicans wouldn't risk 4 years of Hillary Clinton and so they all voted for Trump? No.
Some democrats would prefer a Republican like Trump to an open Marxist, just as some republicans would prefer a democrat to an ultraconservative like Ted Cruz. Sanders can never hold onto 100% of the democratic base, and expecting him to do so isn't realistic.
The thing is though, despite what the media says, Kasich is no moderate. He was one of Gingrich's minions in the '90's, he went after the unions in his state, and is pretty hard right on abortion. I think enough Democrats would look past that "I'm a reasonable moderate" sales pitch he's been pushing since 2016 and come to the conclusion that he's pretty right wing, and thus not vote for him. His support would still more likely come at the expense of Trump, even if Bernie is the nominee.