If Congresspeople can be pushed out of the party or out of the party's favor (and therefore office) by POTUS, then they de facto do not have power.

The best distillation of our current conundrum I've heard is that our framers architected a system where the branches of government would compete (i.e. protect their power) against each other. Today, we have people in each branch willing to forego their own branch's power in exchange for greater party power.