Theory: All political movements need an agenda, or a villain.
Libertarianism is the first type. The average libertarian has a set of proposals he wants – lower taxes, less regulation, legal drugs, etc. – and keeps pushing until they get passed.
The pro-choice movement is the second type. Abortion is already legal, as much as Mississippi doesn’t like it. There’s no new agenda to implement. But there is a villain – the pro-life movement, which can be made appropriately evil and scary-sounding. Likewise, the pro-life movement’s villain is the horrible baby-killing pro-choice movement. Two such groups can be each others’ villains, and so can keep fighting forever and ever.
If there’s neither, a movement whithers away. For example, almost everyone opposes Nazism. Yet, there are no big anti-Nazi rallies in the streets every week. No one calls you to push anti-Hitler petitions, or denounces proposals to add swastikas to the flag. At some point, people just got bored and did something else.
This means, interestingly, neo-Nazis are doing Nazism a huge disservice. There’s no chance of them ever winning elections. And their existence galvanizes opposition to Nazi ideas, by giving everyone a villain to hate. (The one time you do see anti-Nazi protests is around Nazi rallies.) Conversely, if you want to kill a movement, tell everyone they hate to shut up.
The Rationalist Conspiracy 
The Libertarian villain is Tyrannical Big Government. This seems so obvious that I can’t help but wonder if your ignoring it was intentional, to paint Libertarians as reasonable, policy rather than story based folks, in contrast with their political opponents.
Conversely, while abortion is strictly legal everywhere in the US, there’s plenty of variance in state laws regulating it, ease of access, funding, etc. for pro-{choice,life}rs to fight over.
One trick is that the opposition doesn’t actually have to exist, it just has to be believed to exist; for example in the United States you have nationalism sustained by opposition to “terrorists”. Maintaining a convincing illusion of opposition normally requires some resources and media support. But some movements get by with *un*convincing illusions of opposition, ie conspiracy theorists. And the internet has lowered the bar, so that a few trolls can create the appearance of a side with adherents.
Agreed Jim.
I’m actually very skeptical that you can have a mainstream political movement with just an agenda. In so far as Libertarianism is about policy, it’s not mainstream. In so far as it aspires to being mainstream, it degenerates into right-wing witchhunts for ‘welfare queens’ and the like.
I was with you with every word until your last sentence.
“Conversely, if you want to kill a movement, tell everyone they hate to shut up.” What does that mean?
Also, I believe the most iconic figure of political villainy must be Guy Fawkes.
“Remember remember the fifth of November
Gunpowder, treason and plot.
I see no reason why gunpowder, treason
Should ever be forgot…”