  Previous work on voter control, which refers to situations where a chair
seeks to change the outcome of an election by deleting, adding, or partitioning
voters, takes for granted that the chair knows all the voters' preferences and
that all votes are cast simultaneously. However, elections are often held
sequentially and the chair thus knows only the previously cast votes and not
the future ones, yet needs to decide instantaneously which control action to
take. We introduce a framework that models online voter control in sequential
elections. We show that the related problems can be much harder than in the
standard (non-online) case: For certain election systems, even with efficient
winner problems, online control by deleting, adding, or partitioning voters is
PSPACE-complete, even if there are only two candidates. In addition, we obtain
(by a new characterization of coNP in terms of weight-bounded alternating
Turing machines) completeness for coNP in the deleting/adding cases with a
bounded deletion/addition limit, and we obtain completeness for NP in the
partition cases with an additional restriction. We also show that for
plurality, online control by deleting or adding voters is in P, and for
partitioning voters is coNP-hard.
