  In the Shift-Bribery problem we are given an election, a preferred candidate,
and the costs of shifting this preferred candidate up the voters' preference
orders. The goal is to find such a set of shifts that ensures that the
preferred candidate wins the election. We give the first polynomial-time
approximation scheme for the Shift-Bribery problem for the case of positional
scoring rules, and for the Copeland rule we show strong inapproximability
results.
