The phrase "vocal proponent" refers to a person who strongly supports or advocates for something and is not afraid to express their opinion or voice their views openly and loudly. Full definition
«They've been warned that they can't let down the veterans,» said Aaron Bedard, who was injured while serving in Afghanistan and is now one of the most vocal proponents for bringing back the pensions. (moneysense.ca)
«Five years ago,» says Richard Greenfield, a media and technology analyst at BTIG who happens to be Netflix's most vocal proponent on Wall Street, «we wrote a piece saying that the networks shouldn't license to Netflix because they were going to unleash a monster that would undermine their business.» (nytimes.com)