If the overwhelming pattern is that the members of a single party change their views on multiple issues in concert, particularly of those shifts involve disavowal of prior views (or even diametric opposition to views previously held), while those of other parties (not just the primary opposing one), or persons outside that political system (e.g., in other countries) don't, and there's an expressed policy of opposing a party or its head, then I think the suspicion that the shift is largely partisan is rather well founded.