Well they aren't a monolithic party. They're a loose coalition at best. And they've got groups left and right trying to sabotage any solidarity or progress. It makes it pretty tough.
Yes... And most of those saboteurs come from inside the house. Look at all the Pro Palestine people who either skipped the vote because they were mad at the Democrats or they voted against them on this single issue thinking that Trump was somehow going to make the situation over there any better.
I'll never deny that Republicans and the greater right do pretty much everything in their power to block or trip up the Democrats whenever possible, but you also have to admit that the Democrats have their own internal issues that are the cause of much of their own dysfunction.