I'm not familiar with the research you're alluding to. But perceived centrism is one of the few things that we know to be an advantage in elections. Also note that 2016 was a favorable year for Republicans and that Trump underperformed down-ballot Republicans (and lost the popular vote, even with the Comey boost).
If you can't use your brain to imagine a handful of situations in which being a centrist candidate would give you a disadvantage against a more radical candidate, I don't know what to tell you. Let me give you a hint: imagine 100% of the population became radical leftist overnight. It is trivial to note that a radical leftist candidate would beat a centrist candidate in an election. Now you can see that the plausibility of a radical candidate beating a centrist is partially a function of how radical the underlying population is. Hence, there is no law that mandates a centrist candidate is superior. This is contingent on the bulk of the population holding what are known as centrist views, but this need not always be the case. If you believe in a Median Voter Theorem kind of world, if the median voter becomes more radical, it pays to run as a radical candidate.
Voters preferred Clinton to Bernie by a pretty large margin.
The RCP popular vote total was 15,805,136 Clinton to 12,029,699 Sanders. She had 2205 delegates (not including super-delegates) to Sanders' 1846. Not a small margin, but not exactly a mandate for Hillary. You could look at what I'm suggesting in 1 of 2 ways. Either 1) Hillary failed to move to the left to capture voters who needed someone to speak to economic concerns or 2) party elites favored Hillary over Bernie, possibly costing the Democrats the general election against Trump.
But let's remember why we're talking about this. The question posted by the OP is essentially, why did the Democrats become the party of Russiagate and anti-Russian sentiment? My answer is that 1) emphasis on Russiagate is a political ploy to hamstring Trump and 2) at the time of the Russiagate narrative, the election meddling / Russian interference was one of the reasons given for Hillary losing - I'm saying Russiagate is a much more politically convenient narrative for losing, rather than admit that Hillary just missed her opportunity to speak to the economic concerns of the nation by being a centrist instead of moving to the left.
It seems strange for you to assert that Democrats would be harping on and on about Russiagate if Trump had lost, if that's what you're saying. You really want us to believe that the Democrats would spend so much time in session and in the news talking about Russia and Trump if Hillary were in office? That's kind of a weird position to hold. Note, I'm not saying there wouldn't have been an investigation as you seemed to misread, I'm saying the Democrats would not have made this into such a big deal had they won - primarily because there is little benefit from beating a dead horse.
Notice how, after the Mueller report was a nothingburger, they moved on to the next scandal. Disrupting the Trump presidency is a high priority for them.
can sit across from me, in public, no less, and speak with such disrespect."
