If the Democrats start pushing for stuff like that, they're clearly heading for defeat in 2020. Seriously, the Democratic Party hasn't been this messed up since 1972 imo.
They would not be “clearly heading for defeat.” It’s not a good policy, but neither was suggesting a ban on all Muslims coming into the U.S. and yet the guy who suggested that currently sits in the Oval Office. We are past the point where absurd things like this would count as disqualifiers for the general electorate.
Thankfully, no serious Democratic candidate is actually considering banning all Russians from entering the country.