A large percentage of those are overseas, and the remainder are concentrated in Mountain View (yes, I'm aware of satellite offices in SF, NYC, Boston, etc.) Call it 15K HQ employees, which should be enough to make an impact. Right?
Except that their own senator, California's Dianne Feinstein, remains arguably the most vocal NSA surveillance enthusiast on Capitol Hill. She'll get reelected with or without the help of those 15K voters -- in a state with a population of 40 million. (In reality the number is even lower because not all employees are U.S. citizens eligible to vote.)
And those 15K concentrated employees are negligible compared to companies like AT&T, which has 243,000 employees which tend to be inside the U.S. and spread more evenly through congressional districts:
http://finance.yahoo.com/q/pr?s=T+Profile
