When there is not enough popular support for the "best party", and revolutionary overthrow is non viable, then the "true mark of anti-fascism" is knowing when to vote for the least bad party and then continue campaigning and canvassing to pressure for better policy and candidates

Its disgusting and it feels terrible, but (speaking broadly) if the progressive voter base leaves the a country's primary "left leaning" party, then the party will reach instead for center right voters to fill that gap, driving the party further right.

I won't tell you prescriptively who to vote for, but please, give it some thought in good faith