Just because you don’t agree with criticism doesn’t mean It’s not constructive. You asked how a progressive party would work, the type of local grassroots activism and organizing cutthroat describes is exactly how many nascent parties form. You grow your influence where you can when you can as a bloc that others have to win the support of if they want to win. Once your movement is big enough you either capture the party that has become dependent on you or take your base of power and start your own.