Good Point

The nature of coalition politics is whether inside of or among parties, is that the sides compromise , with each side getting some of which it wants.

Matt Bruenig observes, quite accurately IMNSHO, that this is not the case for the Democratic Party, but instead that the conservatives (He calls them the "Center Left" which is not accurate) are literally offering nothing for their more progressive counterparts.

They refuse to deal on policy, and seek to deplatform and remove from office progressive politicians and activists, and pay no attention at all to their policy prescriptions, even when they are overwhelmingly popular.

This is not a recipe for success, but the Democratic Party establishment (There is no Democratic Party establishment) makes its living largely unsuccessfully defending the indefensible, so it is all good for the consultants:

Matt Yglesias has a piece titled Against Murder-Suicide Politics where he criticizes the left for threatening or actually withholding electoral support from Democrats who do not pursue their desired policies.

I should note here my two rules for Matthew Yglesias:

1. Matthew Yglesias is wrong.
2. See rule 1.

Yglesias starts the piece by establishing what he thinks is normal and productive behavior within a political coalition by pointing to how the right-wing of the Republican party operates, which he says is by providing electoral support without requiring that candidates tout their issues on the hope that they will get their issues dealt with when Republicans win.

Then he contrasts that with the way the left-wing of the Democratic party operates, which is suggesting or threatening to withhold electoral support unless candidates tout their issues and pursue them when they are in office, which he compares to murder-suicide.

See rule 1.

To my mind, the normal and productive way to do coalition politics is for the various factions in the coalition to first acknowledge that they disagree on certain things but that it is nevertheless beneficial to work together in order to control the government. From there, the various factions ought to receive policy dispensations that are proportional to their size.

Thus, a small left-wing faction in coalition with a large center-left faction may be allowed to pass a policy or two and get one cabinet position even though the center-left faction may not support the policy or the appointee. In exchange, the left-wing faction will agree to pass the policies and support the appointees of the large center-left faction even though the left-wing faction may not support those policies or the appointees.

You see this kind of arrangement a lot in multi-party democracies during government formation. 

Something like this occasionally happens in the US, but not generally. Instead, the general expectation among Democrats at least seems to be that the left should support the appointees and policies of the center-left regardless of the relative sizes of the two factions and that what the left should get in return for that is the avoidance of right-wing governance. Even if a policy is supported by the majority of the Democratic party, including the Democratic elected representatives, it is seen as perfectly normal for even a very small number of moderate Democrats to veto it, so long as that number is enough to prevent a majority.

Furthermore, the conservative (not moderate) Democrats think nothing of refusing to endorse their more progressive compatriots while demanding lock-step support from them.

What's more, the conservative (not moderate) Democrats have near absolute control of the party apparatus with regard to candidate selection, which is how we ended up with a 2 term Congressman whose only qualification is that they won the lottery.

………

But I don’t think you can understand why the left threatens electoral support without fully appreciating the problems created by a world where the center-left’s [conservative] understanding of the proper role of the left is to provide endless confidence and supply rather than to share in the power.

Needless to say, this policy, which can basically be described as, "Vote for us, you stupid commie pinko cocksuckers," is a sub-optimal way to motivate your base, as is evidenced by the results of the 2016 election, where one of the most prominent and blatant proponents of this methodology lost the Presidential election to an inverted traffic cone.