Perhaps what is needed is a “progressive caucus pre-primary.” Have a party within a party. The progressives hold an unofficial primary in 2027 between anyone who wants to run under the banner. They hold debates, have some way of getting people to vote, etc. The progressive caucus holds debates and selects a single candidate to endorse. Then, going into the actual primary, the progressive voting base is entirely united around one candidate from the beginning. That candidate would also have a hell of a lot of momentum going into the primary as they would already have one wing of the party entirely behind them.