comment: there would be no risk of a spoiler effect.
  
  This isn't true, there are just multiple spoilers in this scenario. Spoiler means somebody whose entry into the race peels off votes from a would-be winner and makes them into a second place finisher. With the right configuration of votes, it is possible for both the progressive and MAGA third parties to be spoilers and cause a Republican or Democratic victory respectively.
  Therefore, were I a progressive, given the current American electoral system, I would not vote for the progressive third party spoiler.