Looking at this map and the poll figures behind them, it seems pretty obvious that anyone living in those states which are shown as dark red or dark blue could happily vote for anyone they like, in the knowledge that this can't make any difference to the result one way or another.

And that's the rationale behind vote swapping - because you can cast that vote for someone you wouldn't normally vote for, on behalf of someone in a marginal state where that vote might make a difference. All it needs is mutual trust, which of course is quite a tall order. But I know people I'd trust in a similar situation, and I'm sure most people do.