Labour's manifesto wasn't hard left (it was basically Thatcherite) but C****n is a freaking Stalinist. Come on guys.
He's also really unpopular. His numbers are worse than Ed Miliband's before the 2015 election.
(
source).
It's a little similar to the 2016 U.S Presidential election - the most unpopular candidate ever is competitive because his opponent is the second most unpopular candidate ever.
You're entitled to your own opinions as to C****n's policies and integrity but it's a matter of fact/data he's a millstone around Labour's electoral chances. Labour could form a government under C****n but it likely requires extremely favourable circumstances (e.g poor economy which is possible) and any governing majority they achieve would very likely be larger under different leadership.
For those who don't think C****n is Labour's political kryptonite:
i) Why do you consider a major party leader with a net approval of -27% an electoral asset?
ii) Why are Tories are ahead in national polls? i.e is it because Tories doing a good job, Theresa May is popular etc?