Used to express explicit political support or preference in a public manner. During the rally, he didn't hesitate to declare for his preferred mayor.
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Standard Elections
'Declare for' is common during election periods when candidates or supporters make their positions known. With the elections approaching, more celebrities declare for their favored candidates.
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Not Always Literal
Can be used figuratively to express support for an idea or movement, not just political entities. The artist declared for free speech through his provocative artwork.