Not entirely true. It depends on their original level of entrenchment. If they are still at the level of "I don't know much about this Obama guy, so I'm for Clinton," a reasoned argument can convince them to switch to Obama.

But in terms of issues, they're virtually identical candidates in what they support and what they're campaigning on. Which means that anyone who's making an argument against one of them (as opposed to FOR one of them) is doing so based on personal opinion of the candidate themselves (rather than where they have stated they will take the country), and nine times out of ten that ends up nasty, rude, bigoted, violent, or egregiously stupid.

But really, my thought process was more aimed towards a general election, where I will wager you that someone who has strong opinions about any two major party planks won't ever cross the aisles, and a "debate" over two candidates will turn into mudslinging over the irreconcilable issues.

Hey, some of us are independent.

Otherwise, yes, if a person identifies as a strong Democrat is in a discussion with someone who identifies as a strong Republican, the fur, it is going to fly.