This excellent article points to the two ways of forming similar but disparate political groups into one.

As presented here, one way is to knit together, via compromises and tradeoffs, an alliance that will advance apace across a broad front. Even if successful, one weakness of that approach is that, at a minimum, those compromises and tradeoffs reduce the enthusiasm of members of all groups.

The other is the principle of convergence. In it a (new) idea that is well to the fore of all groups, but one all groups can endorse, forms a focal point towards which all can enthusiastically advance, subordinating differences as a matter of course.

The problem there is to find such an idea. Perhaps, though, the search for such an idea should be the primary project of 'progressives' everywhere.