There have now been almost a hundred Council by-elections since May and Labour's vote has gone down in all but (I think) five. Why does anyone think they can win the General Election?
Does anybody seriously think they can?
Their only feasible objective on this occasion can be to deny the Conservatives a majority.
I think a lot of Corbynites honestly believe that they're going to win. You have to remember that there are a lot of people who inhabit an electronic echo chamber of pro-Corbyn Facebook groups and alt-left fake news websites.
Although the phenomenon also existed before the social media era - I remember speaking to an old-school moderate Labour councillor who told me that he thought Labour were going to win in 1983. He described how he left a Foot rally feeling as if he was "walking on air" and believing that he was just hours away from a radical socialist government.
There is something about political activists that stops many of them from seeing things objectively, and makes them prone to wishful thinking and outright self-delusion.