Definition of "grass-roots"

Another Bush — George W.’s brother Jeb — is likely to be a big player in the Republican Party’s future. And the most powerful grass-roots force in America, the Tea Party, derives its energy mostly from the same older, white voters who once vilified Bill Clinton.

2012 November 7, Matt Bai, “Winning a Second Term, Obama Will Confront Familiar Headwinds”, in New York Times‎