You're talking about the chronology, and the chronology does matter.
For example, in the case that you mentioned, it would be appropriate for a party to later piggyback on a government initiative, but it wouldn't be appropriate for a government to then continue with that same policy. In reality, of course, all parties campaign on what they've done, so there is a really blurry line between these two. What we need to do is make sure that the distinction is as clear as possible. The way you regulate it is by regulating or controlling how taxpayers' monies are spent.