  Opinion formation is a process with strong implications for public policy. In
controversial debates with large consequences, the public opinion is often
trapped in a fifty-fifty stalemate, jeopardizing broadly accepted political
decisions. Emergent effects from millions of private discussions make it hard
to understand or influence this kind of opinion dynamics. Here we demonstrate
that repulsion from opinions favors fifty-fifty stalemates. We study a voter
model where agents can have two opinions or an undecided state in-between. In
pairwise discussions, undecided agents can be convinced or repelled from the
opinion expressed by another agent. If repulsion happens in at least one of
four cases, as in controversial debates, the frequencies of both opinions
equalize. Further we include transitions of decided agents to the undecided
state. If that happens often, the share of undecided agents becomes large, as
can be measured with the share of undecided answers in polls.
