On 2 January 1988, Margaret Thatcher became the longest continuously serving Prime Minister of the 20th Century, serving for 11 years and 209 days and the only prime minister in that century to win three consecutive terms. On 14 November 1990, former Defense Minister Michael Heseltine challenged her for leadership of the party, partly due to differences of opinion on the European Union. Thatcher won the first ballot but by too small of a margin for outright victory. That night, her cabinet members visited her one by one and urged her to resign. She officially stepped down on 28 November after helping to assure that John Major and not Heseltine would replace her.