I think he has hid it or suppressed it for so long, or at least made sure not to advertise it, that people never knew for sure. & given he started out teaching at Calvin College & affiliated with the CRC (both conservative in general) I assumed he was conservative as well. But this comment makes it quite clear he's not, but quite the opposite:
"Richard J. Mouw who is President emeritus and professor of faith and public life, Fuller Theological Seminary, Pasadena, California, shared, "Given my political preferences, my feelings right now border on despair. But I also know that people whom I admire—even love—voted for Mr. Trump. How do we talk together now about the deeper issues without mutual accusations? What hopes and fears accompanied us into the voting booths? How do we love each other in continuing to work together in the service of the only true and righteous King?"
http://www.gospelherald.com/articles/68021/20161117/trump-presidency-victory-response-christian-leaders.htm
Vk
http://www.gospelherald.com/
Vk