While the rank and file among the Democrats is full of True Believers, Fellow Travelers, and Useful Idiots, the career Democrat politicians are something quite different. And, due to that fundamental difference, they blame President Trump for everything and anything, especially those things they, themselves, did or brought into being.
These sorts are career Democrats and they’re just doing their jobs as they seem them. Understand that they think, not completely wrongly, that their jobs are to stay in office. Anything they do in the course of that is important to them only in the context of that sole purpose.
Hence, since blaming President Trump and vilifying him seems to resonate with their constituencies, they consider doing that essential to their jobs.