Who is Saul Alinsky? 

Republican presidential candidate Newt Gingrich knows. And, Gingrich thinks, Democratic President Barack Obama does, too. Gingrich compares the two, in fact.

Gingrich, who won the South Carolina Republican primary on Saturday, has repeatedly invoked Alinsky's name recently. 

Saul Alinsky radicalism is at the heart of Obama, said Gingrich on Sunday, in an interview with CNN Chief Political Correspondent Candy Crowley.

That's not the first time Gingrich has melded the names of Obama and Alinsky, either.

Said Gingrich on Saturday in a victory speech in South Carolina: The centerpiece of this campaign, I believe, is American exceptionalism versus the radicalism of Saul Alinsky.

Gingrich had mentioned Alinsky before that, too. On New Year's Eve in Iowa Gingrich said, according to CNN, that Obama really is sort of a classic Saul Alinsky radical whose basic ideas are the opposite of what we need to create jobs.

Saul Alinsky, the activist and writer whose street-smart tactics influenced generations of community organizers, most famously the current president, could not have been more clear about which side he was on, according to The New York Times.

Alinsky, born in 1909, died in 1972. He is considered to be the founder of modern community organizing, and Obama, of course, is known as being a community organizer. 

Alinsky is known for his 1971 text Rules for Radicals. 

What follows is for those who want to change the world from what it is to what they believe it should be, wrote Alinsky. 'The Prince' was written by Machiavelli for the Haves on how to hold power. 'Rules for Radicals' is written for the Have-Nots on how to take it away.

Alinsky was known for pushing news ways to organize the power and so-called powerless, and such positioning was both controversial on one side and hailed on another.

To learn more about Alinsky's views from his positions in Rules for Radicials click here.