Joseph Chamberlain

Formal switch. The radical Liberal mayor of Birmingham resigned from Gladstone's cabinet in 1886 over Irish Home Rule and led the Liberal Unionists, who allied with the Conservatives and eventually merged with them in 1912. He became Colonial Secretary in Salisbury's government and later split that coalition too, with his 1903 campaign for tariff reform. Few individuals have broken two major parties, and the Liberal Unionist split kept the Liberals out of office for most of the next twenty years.