A "vocal advocate" is someone who openly and strongly supports and defends a particular cause, belief, or idea. They speak up and passionately express their views on a topic. Full definition
The students have become vocal advocates for stricter gun laws. (sun-sentinel.com)
In our field visits, we heard stories of teachers who were once harsh critics of PL becoming its most vocal advocates. (educationnext.org)
I am a very vocal advocate of attachment parenting and the consequences that impact our lives if we choose not to fully attach to, and be attached to by, our children. (attachmentparenting.org)