proponent

I've long been a proponent of term limits for pundits and columnists.

Vincent St. John was also interested as a proponent of this plan.

He also is a proponent of the idea that hypnosis is an abnormal manifestation of a normal process, an opinion he shares with many.

In all debates the proponent has the right of opening and closing.

If this bald statement required elucidation or expansion, its proponent didn't seem to realize the fact.

That is when he wrote it, and he let the proponent in this case have it.