Voting third party right now also just perpetuates both parties. There are enough people in this country to elect anyone from the major parties, so a third party can’t win unless one of those parties collapses. The only way a party collapses is when it consistently loses elections.

The republicans won’t consistently lose elections as long as progressives don’t vote for democrats, so both parties will continue on. The majority of the people in this country are left of center, so the only way republicans win is by suppressing votes, and one of the ways they do that is by propping up progressive third party candidates.

If we truly want a progressive party, making sure republicans never win elections is the way to do it. Then either the Democratic Party will shift left and republicans will regroup under a new less extreme conservative party, or the Democratic Party will shift right as it absorbs all the republicans and a new progressive left party will rise. Both ways result in a more progressive set of major parties.