You can't really say "spoiler effect" without implying it. A "spoiler" is a minor candidate that is substantially similar to one of the major candidates. Just because the Democratic candidate lost to the spoiler doesn't make the spoiler less of a spoiler; the Democratic candidate would have been more likely to win if they weren't there.

It's not a "spoiler effect"; maybe a split constituency is a better term. Spoiler implies that somebody has something, and someone else steps in and ruins it. These are all equal people running for the same office.