The Business Hierarchy
"Normal world" ideas of an agent.
Well, there are Hollywood-style agents. That's what most of the imagery is directed towards.
There are freelance political operatives who tend to do the dirty work, but are independent enough to take on multiple clients. These are different from campaign operatives. They are "consultants." "Advisors." In many ways they would like to be fixers. However, they aren't quite powerful enough, or connected enough, or just ... enough (financially, or skillful).
What marks the difference between agents and fixers is that fixers direct their efforts from the top-down, whereas agents represent people on both sides of the divide, but most of the time they end up representing people to the powerful. They are supplicants by proxy. In the politician's example, the agent still has to request money from donors, although they may not have to ask very hard. Fixers, on the other hand, don't have to ask. They lead by example, and give large amounts of money and/or in-kind donations. An agent will ask the union to send volunteers. Fixers tell them to mobilize.
They are, in many ways, wannabe-fixers. They are the junior varsity players. In many instances, fixers will direct actions through agents and onto the bagman, and eventually the operative.
Finally, the bagman is the bottom of the power broker hierarchy. They are the one who are given orders, and who hire the talent. They are never to be trusted, and are the ones who tend to get shit on because their employer decided to double cross the asset. They don't exist. They don't know shit. Don't want to know shit. They have no need to know.