Wherever Newt Gingrich goes these days – stumping in Florida, arguing on televised debates with fellow Republican presidential hopefuls, jotting down notes for his umpteenth book – he carries with him a scary but useful ghost: Saul Alinsky.

The radical community organizer (gone now these 40 years) is the specter on which Barack Obama has modeled his life, Mr. Gingrich warns. It’s no coincidence, he says, that both Alinsky and Mr. Obama were from Chicago or that the president passed up far more lucrative possibilities to become … a community organizer.

“The centerpiece of this campaign, I believe, is American exceptionalism versus the radicalism of Saul Alinsky,” Gingrich said in his South Carolina primary victory speech, a charge he finds many ways to repeat. "Saul Alinsky radicalism is at the heart of Obama,” he said on CNN.

Election 101: Ten questions about Newt Gingrich as a presidential candidate

So who was Saul Alinsky?

Born in Chicago in 1909 to Russian immigrant parents, Alinsky worked his way through the University of Chicago, then dropped out of grad school to organize the poor in the city’s slums, demanding better working and living conditions. He went on to do the same thing in other US cities.

Published the year before he died in 1972, Alinsky’s “Rules for Radicals: A Pragmatic Primer for Realistic Radicals” has been compared with the writing of Thomas Paine, and it inspired many young idealists (including, apparently, Hillary Rodham Clinton, who wrote her Wellesley College senior thesis on Alinsky).

"What follows is for those who want to change the world from what it is to what they believe it should be,” Alinsky begins his book. “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."

Some compare Alinsky’s activities and goals with a more recent American political insurgency.

“The Tea Party comes from the same sense of outrage that the elites, as Gingrich calls them, are running the country,” Dick Simpson, a University of Illinois at Chicago political scientist and former Chicago alderman, told Bloomberg News. “The Tea Party has understood how to mobilize their anger and turn it to political results, which is the underlying motif of Alinsky.”

Alinsky, Mr. Simpson says, was “a master community organizer who attempted to organize people without power, people that today we’d call the 99 percent, by using the strength of numbers to overcome clout and wealth.”

FreedomWorks, the tea party group headed by former Republican House leader Dick Armey, gives copies of “Rules for Radicals” to its leaders. “His tactics when it comes to grass-roots organizing are incredibly effective,” FreedomWorks spokesman Adam Brandon told The Wall Street Journal. Tea partyers aggressively confronting lawmakers at town hall meetings is straight from Alinsky’s playbook.

Conservative icon William F. Buckley Jr. called the Chicago radical "very close to being an organizational genius."

As a former history professor, Gingrich not only understands Alinsky’s motif, he’s made it a key part of his campaign.

“Gingrich's clashes against the establishment are classic Alinsky,” writes Philip Klein, senior editorial writer for the conservative Washington Examiner.

Democrats may rankle at Gingrich’s painting Obama as an Alinsky acolyte – a sort of red-baiting, although Alinsky never joined any organization, communist or otherwise. But some Republicans say the former House speaker himself is destructively channeling Alinsky when, for example, he goes after Mitt Romney’s wealth.

“What the hell are you doing, Newt?” asked an incredulous Rudolph Giuliani, former New York mayor and one-time presidential candidate, on Fox News.

“I expect this from Saul Alinsky. This is what Saul Alinsky taught Barack Obama, and what you’re saying is part of the reason we’re in so much trouble right now,” Mr. Giuliani said.

All of which makes for a very interesting development within conservative American politics, including the Republican Party.

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“GOP nomination fights are often described as battles between Rockefeller Republicans and Goldwater Republicans,” writes Mr. Klein. “In 2012, Gingrich has brought us the Alinsky Republican.”

Election 101: Ten questions about Newt Gingrich as a presidential candidate