The phrase "outspoken opponent" refers to someone who openly expresses their disagreement and criticism towards something or someone. They are not afraid to speak their mind and voice their opposing views. Full definition
Instead, he had proposed an ambitious plan for free higher education, unveiled alongside Sen. Bernie Sanders of Vermont, one of the country's most outspoken opponents of charter schools. (politico.com)
In its first round of endorsements, ESPA announced it would be backing Charlie Ramos against Sen. Ruben Diaz Sr., the Bronx Democrat who is one of the most outspoken opponents of gay marriage and has already been targeted by Fight Back NY and the Marriage Equality New York PAC. (nystateofpolitics.com)
Well, for one thing, because I have been a very outspoken opponent of Kindle Select in the past. (guidohenkel.com)