There's so much here to unpack.
Firstly, which one is it? Is it communist, marxist, socialist or progressive? Because they all mean incredibly different things. I can also promise you that it's not a joint effort from all four camps, because trying to get factions of the left to work together is like trying to make a herd of stray cats perform a magic trick, it ain't happening.
The USSR, East Germany, Cuba, Vietnam and Cambodia never tried socialism, they tried communism. Communism sucks balls. Not in the good way either. It's a shit, unworkable system which rewards corruption. None of the major players in the Democratic Party, not a single one, is anywhere near what could be described as communist.
Venezuela could be argued to have tried to nearest thing to pure socialism, albeit a heavily authoritarian version. Anything authoritarian will naturally also suck balls.
Italy has never been anywhere near implementing socialism, they're famously right leaning. Fascism and all that?
Scandinavian countries have never tried socialism. They're Democratic Socialists, which is a markedly different thing. And, as it happens, they're all doing rather well. Don't believe the silliness about Sweden, it's an incredibly stable and successful country.
What you think is radical is par for the course almost everywhere else in the world. Some of the Democrats you're concerned about would be called right wing in most of the political world.
Trump isn't a massive threat to democracy, you're right. Those who view him as so are looking at the small term bump rather than the long term movements worldwide. He is dangerous, we've seen that this week, breathing new life into ISIS and turning our backs on the allies which helped us contain them, but there are checks and balances in place to prevent him tearing down institutions. He is also exponentially preferable to Pence, who is genuinely terrifying.
The furthest left candidates standing are Sanders and Warren. They want to give Americans what the rest of the civilised world already has, healthcare for all with nobody going bankrupt over medical bills, a fairer benefit system which actually helps people to help themselves and to narrow the monumental gap between the rich and poor.
I struggle to think how any of those could be seen as a bad thing.