http://knowmore.washingtonpost.com/2015/03/12/chart-the-decline-of-sideburns-and-the-spectacular-rise-of-clean-shave-men/

study linked in other link

http://www.uvm.edu/%7Epdodds/files/papers/others/1976/robinson1976a.pdf page 1136 has the key graph

world war 1 and 2 had a pretty major impact on getting a generation of americans to keep a clean shaven look forced on them by the military.

for this i'm assuming politicians aren't any different from the normal man.

interestingly the safety razor didn't impact this trend (invented 1905)

I responded to this very question a few months ago in this thread in a little more depth than I normally see.