From Longman Dictionary of Contemporary Englishpower baseˈpower base noun [countable] SUPPORT A PERSON, GROUP, OR PLANan area or group of people whose support makes a politician or leader powerful the party’s traditional power base
Examples from the Corpus
power base• The Conservatives suffered the biggest reversal of fortunes, losing two seats in their North Down power base.• Providing information that reduces uncertainty is a second way a department can increase its power base.• McClardy's power base is among working-class Catholics.• But middle-class flight destroys a school system because when middle-class parents flee, so does the power base.• Politicians who have their power base in these areas, including several presidential candidates, are also implicated. 