proponent

a person who advocates a theory, proposal, or project.

synonyms:
advocate · supporter · upholder · exponent · promoter · adherent · endorser · champion · defender · backer · subscriber · patron · espouser · friend · apostle · apologist

Donald J. Trump is a proponent of a wall along the southern U.S.A. / Mexico border.
Our president is a supporter/endorser/defender/champion/backer of a wall along the southern U.S.A. / Mexico border..

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