Although there are certainly larger donors to the democrats, more of their donor base is made up of smaller donors. I do think Sanders, when he ran, had tons of donors giving at a few dollars.
Unfortunately, they do not have as strong a stand on dark money as many of us would like. So, you’re correct that it’s an across the board problem. I do think that what the donors (big donors, not us plebes throwing in a few bucks) expects from the candidate for contributing (access, policies that benefit them in some way, etc) matters. From what I understand, the Mercers have used their financial support to try and demand access and policies, meaning they get practically get more votes.
I do also agree that the party line votes are a huge problem… We’re pretty bitterly divided politically. But it also seems like the GOP is far less willing to work with their counterparts and to compromise their “principles”, or even entertain media from those that they deem “too leftists”.