1990 Eastbourne by-election

The vacancy came from tragedy: Conservative MP Ian Gow was killed by a Provisional IRA car bomb in July 1990. The Conservatives had held Eastbourne by more than 16,000 votes in 1987, yet on October 18, 1990, the Liberal Democrat David Bellotti took it with 50.8%. The shock is credited with contributing to the end of Margaret Thatcher's premiership the following month, because Conservative MPs began to fear for their own seats if she stayed. Few by-elections have a more direct line to a change of prime minister.