The idea that moderates or "the center" would win an election just broke Labours back in UK. It's also what broke Clintons campaign in 2016. The left has a big enough voting base to win elections, what it lacks is motivating them to vote. The exact opposite of what happens when you water down your positions in search of "moderates".

Interestingly, right-wing parties understand this quite well. If they loose an election you never hear that they have to convince moderates. Instead they get even more radical.