Let me ask you this....
Go listen to GOP talking points for a week. Not the campaign stuff, but the current elected leadership in power. Listen to what these guys say. Then come back and tell me how much of it you think they believe when they go home, and how much of it is rhetoric that's necessary to get votes.
When you've done that, then come back and let's analyze the rhetoric of the mid-1800s.
Well, when reading period northern political rhetoric, one can see two trends, an unpopular one and a popular one: the abolitionist rhetoric talking about ending slavery right here, right now, liberty for all, irrepressible conflict, etc., and the more middle-of-the-road rhetoric, talking about compromise, respecting property with the fugitive slave act, just stopping the expansion of slavery but not ending it.
One of those two would definitely get more votes than the other. In fact, Seward probably lost the Republican nomination because he talked too much about abolition, while Lincoln won it because he was more middle-of-the-road, and had backed off from things like his "house divided" speech.
So if you're implying that people said things they didn't really believe, just to get votes, I'd say yes, absolutely. Northern politicians realized that speaking openly about nationwide emancipation would make them lose races, regardless how they really felt. (I don't think most wanted it, but it was political suicide to say you did, if you did.) Most didn't have any idea that the momentum behind abolition would pick up so strong, so fast, so they saw no need to jump on that bandwagon for political reasons.
But in the north, it
was pretty much just political rhetoric, all with an eye toward the ballot box. The outspoken abolitionists being threatened and beaten up and having their property vandalized in northern cities--the brave people who were actually taking action to match their beliefs--were the tiny minority.
As far as what the south believed... well, it wasn't just rhetoric down there. They actually took action too. Secession declarations weren't written to get votes from anybody--it was too late for that. They were preparing for war and preparing to put their careers and lives on the line. So when they said they needed to secede, because the new northern power base wouldn't stop until all the slaves were emancipated, not only did history prove them right later, but they were fitting their actions to their words at the time.