This, I believe, is actually always the case. While swing voters decide a few elections, most elections are decided by how motivated each party's voters are to actually get to the polls.

A great quantitative example of this is in the (somewhat dry) "Rise and Fall of the American Whig Party" by Michael Holt. While it talks about a very different time, the demonstration that the party that got the most of its voters to vote always won was pretty convincing.