I think the point was that any cohort with a strong voting record for one party will still have members that vote for another party based on a single issue. It

I think the point was that any cohort with a strong voting record for one party will still have members that vote for another party based on a single issue. It's not unreasonable to think that someone might agree with the bulk of a party's platform, but disagree on a single issue that's most important to them and can't bring themselves to vote for anyone in that party because of it. I can imagine guns, abortion, the military, taxes, etc. might fit that bill as someone's single most important issue.