If we can talk about the self-reenforcing cycles of emotional abuse cause by creepy teen novels, we can talk about novels as political shills. While I doubt any one book series shifted an election, it's part of a larger trend of politically influenced works that appeal to young readers and encourage mindsets, and the 2000 election was really really close. If the Gung-ho message of the military sci-fi genre shifted the vote by as little as 550 people, we could have had an entirely different president in 2000. I think things much less influential then what you read can shift that many votes, once you blow it up to a state level, never mind a national one.
The same goes for Twilight, despite it's popularity. It still only reaches a small minority of the population in a big way.
There's lots of creepy romance outside of Twilight, and lots Military Sci-Fi outside of David Weber.