I consider myself left wing but the 'all Tories are evil' line the guardian runs with is one that turns me right off.
It's a problem of the left. We all too often start from the premise that Tories are evil, and do what they do solely for the purpose of enjoying the suffering of others. But they regard us as well-intentioned naifs who are wrong even if our hearts are in the wrong places.
If you follow that through to a campaign position, it results in yelling at people who we want to vote for us that unless they repent from the evil (as opposed to the mistake, or the misapprehension, or carelessness) of previously voting Tory they can just fuck off. We don't just want swing voters' votes, we first want them to apologise. And people won't do that. They will say "I voted for Thatcher in 1983 because I wanted to stay in the EEC and NATO, but I voted for Blair in 1997 because I wanted a fairer society, because when the facts change, I change my opinion". Of course, in the new 2016 world, quite a lot of Labour's young support now want an apology for voting for Blair as well, thus yet further reducing the pool of people pure enough to be allowed to vote for Labour.
It's like going to a Bruce Springsteen concert and being quizzed by the bloke you're stood next to about your opinion of obscure outtakes from bootlegs, when all you want to do is hear what Dancing in the Dark sounds like and then Springsteen starts playing obscure outtakes from bootlegs anyway
So the Tories are happy to hoover up Labour votes: "of course you voted Labour in the past, that's fine, but we have a much worse better offer you might like to consider". While Labour start from "REPENT NOW! PLEAD FOR FORGIVENESS! SACKCLOTH AND ASHES! Now you can vote for us". And people don't do that. And if you assume your political opponents are pure evil, you assume anyone who supports them is also evil, and that the only reason for their manifesto is the promulgation of evil; that prevents any sort of rational debate about policy. Labour's last couple of campaigns have been bereft of positive proposals, they have just started from the opening position that the Tories are self-evidently wrong about everything, and once people are told that they will instantly realise the error of their ways.
And then we get beaten. Heavily. Again.