This (the statement that as a volunteer he didn't have power over her), along with "why didn't they say something sooner" is making me seethe. This is why #MeToo is a movement. This is why women are finally speaking up. A volunteer, who in this case may have been hoping to start her career, absolutely would see the politician as a person of power, a person who could make or break her start into a political career. And why would one of them have gone back? Because maybe she blamed herself for what happened (classic victim response), and maybe she really really wanted to get into politics so she thought it would be different the next time, and then she promised herself she just wouldn't stand next to him and wouldn't go into a room alone with him and so on and so forth. Sometimes even though you may not work directly for the person, you find yourself at many events with these creeps because your paths keep crossing because of your job, so you have to shake his hand at a public event (or worse, he greets you with a hug or a kiss) because you need his political support.
Looking at these situations from the outside or in hindsight, it's easy to say "why didn't she ....", but at the time the person, especially a young person, doesn't know how to react, thinks maybe she was giving off the wrong signals, thinks it was just a one-time thing. Until this has happened to you, or to your daughter, or your sister, or even your son or your brother, it's easy to sit back and judge. But people in power take advantage of that power, in all sorts of subtle and not-so-subtle ways. Predatory behaviour is just one of those ways.
As for the comment that with one of the accusers, it was a "failed attempt", it was not. If it happened as described, it was sexual assault.