The big source of cash on the left is old money and dedicated philanthropists. Many many special interest causes are just chock full of spending from a relative few well endowed foundations. With gun control, most "violent policy centers" are endowed by liberal trusts like:
The Annie E. Casey Foundation, The David Bohnett Foundation, The David and Lucile Packard Foundation, The Funders' Collaborative for Gun Violence Prevention, The John D. and Catherine T. MacArthur Foundation, The Joyce Foundation, The Overbrook Foundation, and The Richard and Rhoda Goldman FundCompare this to the NRA which is funded by $20-25 donations and magazine subscriptions from working stiffs like me. But there are a lot of working stiffs like me. Several million actually. Which is why the NRA has several times more money right now than the lefty trusts combined. It is also why the NRA has more votes than the lefty causes too.
You see the NRA is actually a grass roots organization. It has millions of members who care enough to buy a membership (and many other things the NRA sells) and vote in elections. Although their policies may have some grass roots support, the foundations are not grass roots organizations. They're generally endowed trusts where the money comes from investments made a long time ago in board rooms far far away. At some level the NRA must reflect the will of its members or the money stops. The Foundations must only reflect the will of the foundations officers and no one else. Which do you think is more in touch with the person on the street?