Why is the Democratic party often a Circular Firing Squad?

It's a very big Tent Party.

What I mean is that the Republican is very Lock Step (for better or for worse) whereas the Democratic party is sort of "Everything else including the kitchen sink" in American politics.

So, the Republican party is sort of now Trump's Gestapo Fascist Party (much to the dismay of real Conservative Republicans who are principled and who actually helped win World War II and all the wars the U.S. has been in along the way.

So, for principled Conservative Republicans, Trump is the worst thing to hit their party since the KKK "Klu Klux Klan" of the early 1900s in their politics then. There have even been KKK Republican presidential Candidates in the early 1900s too.

However, instead of being "Lock Step" like the Republican party, the Democratic party is incredibly diverse actually encompassing maybe where 5 to 10 parties should be if this were another country other than the U.S.

So, the diverse nature of the Democratic party tends to create Circular Firing Squads often which can really interfere with actually running the country in an effective way, especially for a president like Biden who wants all factions of the Democratic party to like him.

So, his wanting to pull the party together actually has worked against him a lot in this presidency and could prevent him being re-elected in 2024 along with his very advanced age.