[EM] Filling Unscheduled Vacancies With PR

[Alex Small]

Say that a legislator resigns or dies in the middle of his term.  If we
are electing the legislators from single-member districts then the way to
fill the vacancy is pretty easy:  Use the same method as for regular
elections (except perhaps making modifications if the lack of partisan
primaries creates a large field of candidates and the election method
works best for smaller fields, but that's a separate issue).

Now, suppose we elect the legislators via PR.  How to fill the vacancy?

If we just hold a standard single-winner election and the resigning
legislator is from a smaller party, it's almost certain that the
replacement will be from a larger party.  The whole point of PR is to give
the various portions of the electorate representation according to their
size, so filling the vacancy in this way upsets proportionality.

(At this point somebody will probably point out that organized political
parties are not the only way to divide up the electorate, so we should use
candidate-based methods rather than party lists.  While I more or less
agree with that view, in theoretical discussions the concept of a party is
still useful.  Implicit in proportionality is that various ideas and
factions are represented according to their size, so we need to think
about these factions to see if the method is achieving proportionality,
and the word "party" is as good a way as any to label those factions.  So
bear with me here.)

Now, if we used party lists then the resigning legislator could simply be
replaced by somebody from his party, perhaps whoever on his list got the
most votes without being elected (in an open list method).  But if we use
candidate-based methods (e.g. STV, PAV, cumulative voting, etc. etc.) then
we're saying that, while political parties will undoubtedly exist in
practice, they have no fundamental role in the design of our electoral
system.  So it would seem dubious to let party leaders pick the
replacement.

What do people here think is the best way to replace legislators in the
middle of a term while retaining proportionality?  What solutions have
various countries devised?



Alex