I like this idea, but it needs to be very broad, primarying key players that represent the old guard. If the people at the top don't want a socialist to succeed, they'll sabotage their campaigns.

An American example would be the way the Democrats undermined Sanders' recent presidential run, or the way "the squad" is treated by the wider party.

A more damning example would be what the Labour party in the UK did to their own leader, Jeremy Corbyn: leaking lies to the press, disrupting funding for their own races, etc. The right wing of the UK's "left" party basically sabotaged their own party's campaign 'cause they couldn't bear to let a socialist win. Now they have a new leader who (surprise surprise) supports nearly every Conservative policy.