I volunteered for Bernie Sanders. His two runs for President (along with a long career) are probably as close as you can find to what a modern progressive party would look like.

https://www.opensecrets.org/pres16/candidate?id=n00000528

He raised a lot of money, had very large rallies, and a lot of very passionate volunteers. But lost, and there’s two reasons why.

1. First past the post spoiler effect - Bernie had to run as a Democrat within the Democratic Party primary system. If he had run as progressive or democratic socialist he would have split the democratic vote. In a first past the post system Duverger’s Law mathematically guarantees 2 party rule.

Any progressive alternative would split the democratic vote, and ensure that, at least for a while, the republicans would win every election. You can see on Lemmy and Reddit and all other kinds of social media the amount of anger and infighting this causes on the left. This is a strong disincentive for anyone to start an alternative party.

2. The donor class - the Democratic Party is largely funded by big money donors. Big money donors have a lot of money because of how things are currently arranged. If the way the country works today has made you fabulously wealthy, even if that means a lot of people suffer, you tell yourself “they suffer because they don’t work hard like me” and want things to stay the way they are. So you donate to both parties to control them and make sure that whatever particular apple cart you’ve cornered doesn’t get overturned.

Every problem the American people face is a profit generator for some fuck face. Rent too high, some landlord is enjoying record profits. Can’t afford medicine, some pharmacy CEO is buying their third yacht. Those people have enough money to buy politicians, ads, political parties, media networks, social media companies, etc. They aren’t just going to sit back and let you fuck up their money making machine, they will deploy those assets against anyone that threatens the status quo.

Here’s a particularly egregious example coming from MSNBC during Bernie’s last run when his reforms threatened their wealth https://www.rollingstone.com/politics/politics-news/chris-matthews-bernie-sanders-public-executions-949802/

So that’s what any progressive party is up against. The mathematical certainty that they would lose until they could unseat the current Democratic Party, something that would take some number of election cycles. The donor class wanting to thwart any change. And let’s say they do overcome both of those things. That party then becomes the thing the donors try to buy next. Your party starts with high minded ideals but one by one the members of your party get big paydays from the billionaires and suddenly they want to soften this reform and maybe hold off on that reform and… oh look they are holding the exact same positions as the current Democratic Party. Because those positions are the positions of the people that own the party, and they will happily buy another.

Campaigning in the US relies heavily on money from wealthy investors to get off the ground. Meaning, any new party that wants to get going needs approval from the wealthy to do so.

Additionally, a huge percentage of the population pays no attention to politics at all, just closing their eyes to the whole election and either not voting, or voting for the party they’ve always voted for every time, so even if your party managed to get some attention, it’d just be another 3rd party further fracturing what small portion of the population risks voting outside the 2 party system as it is.

In other to have a shot at winning, you’d need to somehow make enough money to afford competing with the 2 established parties for screen time, which would mean major corporate backing that would only happen if they liked your policies.

There’s also the choice of doing what Bernie did, and build up an alternative from the local level, but that would require people to realise that politics aren’t restricted to TV-level races nor snooze for 4 years.

If Americans did that in large scale they could to the democratic party the reverse of what the tea party did to the republican party.