The action of adopting or supporting a belief, cause, or idea; or the act of marrying and the relationship of being married.
From Middle English espousal, derived from espouser meaning 'to marry' and later 'to adopt.' The word shifted from exclusively meaning marriage to include the sense of adopting ideas or causes.
Espousal is brilliant because it works the same way in both romance and politics—when you 'espouse' a cause, you're metaphorically marrying yourself to it, treating it with the commitment of a spouse!
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