Los Angeles (CNN) Roger Stone loves resilience. It's why the former body builder had Richard Nixon's face tattooed on his broad back.

"It's there to remind me that in life, when things don't go your way, you get back in the game," Stone said in an interview with CNN. "Nixon said, 'A man is not finished when he's defeated, he's only finished when he quits.'"

When it comes to Donald Trump's 2016 presidential bid, things did not initially go Stone's way. He had one vision for the campaign; Trump had another. But after leaving in August, Stone is back, in a manner of speaking. With the Republicans potentially facing a contested convention, his brand of political trench warfare is now in greater demand than ever.

Late last month, Trump appointed veteran GOP strategist and lobbyist Paul Manafort -- Stone's longtime friend and business partner, dating back to the Reagan years -- to lead his fight for delegates. Sources close to all three men say Stone played a role in that appointment, which gave him a new lifeline into Trump's campaign.

Stone also heads "The Committee to Restore America's Greatness," a pro-Trump super PAC that has redirected its mission "to help stop the Republican establishment from stealing the Presidential nomination" from Trump -- which, of course, will be the campaign's chief preoccupation between now and the Republican convention in late July.

The campaign changes come as Trump has repeatedly charged that the nominating process is "rigged" to block him, a statement Republican National Committee Chairman Reince Priebus dismissed as "hyperbole" in an interview with CNN's Dana Bash on "State of the Union."

But Stone's most significant role will likely take place this summer in Cleveland. In a contested convention, his mastery of political dark arts could prove instrumental in securing the delegates that Trump needs. He has been to every Republican convention since 1964, and he's worked the floor at every convention since 1972. And even he readily admits that he is capable of employing tactics other operatives wouldn't dream of, let alone try.

While Trump and his campaign can claim no connection with Stone -- after all, he left the campaign last August -- those who know the two men say that they speak regularly, and that Stone is an influential voice in Trump's ear.

"Roger is never too far away from Trump ... He's always talking to Donald," a source close to both men said. "Roger and Trump always wind up finding their way back to each other," said another.

Of his contacts with the front-running candidate, Stone says, "I talk to Trump from time to time, but not every day. I don't even necessarily talk to him every week."

Rivals criticize Stone's involvement

Stone's resurgence worries Trump's competitors, who appear to fear his role in the delegate fight and at the convention. Last week at a CNN town hall meeting, Senator Ted Cruz said Stone was "pulling the strings on Donald Trump. He planned the Trump campaign, and he is Trump's henchman and dirty trickster."

which ultimately led to his resignation as Governor of New York. The "dirty trickster" charge is one Stone is familiar with: He was Nixon's "dirty trickster" before he was Reagan's "dirty trickster" before he was George H.W. Bush's "dirty trickster." In 2000, he helped George W. Bush in the Florida recount effort by working with local media and Spanish-language press to amplify pro-Bush efforts to shut down the recount in Miami-Dade County, and he later claimed to have obtained the frequencies of walkie-talkies that Democrats were using to communicate so he could listen in on their plans. He has also claimed that he was first to learn about Eliot Spitzer's affairs with call girls,which ultimately led to his resignation as Governor of New York.

"One man's dirty trick is another man's civic participation," said Stone.

"He's the type of the guy who doesn't really pull any punches," said Tony Fabrizio, the veteran Republican pollster who is close to Stone. "Most people have one of two reactions to Roger: They either love him or they hate him. There's no middle ground with Roger. He is just the type of guy who generates heat everywhere he goes."

Stone was recently blacklisted from both CNN -- for disparaging remarks he made about CNN political analyst Ana Navarro -- and from MSNBC because of what the network described as his "very well-known offensive comments."

He was born in 1952 and raised in Lewisboro, New York, about an hour north of Manhattan. He read Barry Goldwater's "The Conscience of a Conservative" as a child and by the time he turned 13 he was volunteering for William F. Buckley, Jr. George Washington University brought him to the nation's capital, and in 1976 he was named national youth director for Reagan's first presidential bid.

Reagan is how Stone got introduced to Trump: In 1979, Stone came to New York to help Reagan's longshot effort to win the state's primary. "New York was Bush country," Stone said. "Trump let us use his plane, let us use phones, let us use office spaces he didn't even own ... We got to be friendly, and in 1981 when we founded Black, Manafort and Stone" -- Stone's former lobbying firm with Charlie Black and Manafort -- the Trump organization was among our first clients." In 1999, Stone helped lead Trump's presidential bid on the Reform Party ticket.

Sixteen years later, Trump again called on Stone to play a key role in his campaign. But the formal assignment was short-lived.

At the outset, Stone advocated for traditional methods: polling, analytics, advertising. But Trump had something different in mind: hold rallies, generate controversy, get free media coverage. Stone didn't care for that approach, nor the man tasked with implementing it: Corey Lewandowski, Trump's campaign manager, who didn't much care for Stone, either, sources close to the campaign said. (Both Lewandowski and Trump spokesperson Hope Hicks declined to be interviewed for this story.)

So Stone left in August, less than two months after the campaign launched. But he never really was gone. He was not ousted, as was originally reported, nor was he forced into exile, as some journalists would claim. He was always there, on the sidelines, talking to Trump on a regular basis, planting stories in the press, influencing things where he could, several sources said.

What will Stone do next?

Now, eight months later, Trump's "say anything" strategy has given way to a new phase. He's trying to assemble the 1,237 delegates he needs to clinch the nomination, and he's in desperate need of experienced political infighters who can navigate the contentious fight for delegates. Which means that Stone's services are back in demand.

"If it is exile," Stone said of his predicament, "it's Elba, not St. Helena."

In fact, if you buy the Napoleonic comparison, Stone is already marching on Paris.

Cruz fears Stone's next "dirty trick" involves inciting Trump supporters at the convention. Earlier this month, Stone called for "protests" and "demonstrations" in Cleveland, and said he would "disclose the hotels and the room numbers of those delegates" who were involved in "stealing" the nomination from Trump.

Though Stone never explicitly called for violence against any turncoat delegates, Cruz, among others, read it as a threat. The Texas senator called on Trump to fire both Stone and Manafort -- another indication of just how closely those two men are aligned.

"I don't know if the next thing we're going to see is voters or delegates waking up with horses heads in their beds," Cruz said on Dana Loesch's show on The Blaze. "This doesn't belong in the electoral process. ... [Trump] needs to fire the people responsible. ... He needs to denounce Manafort and Roger Stone, and his campaign team that is encouraging violence, and he needs to stop doing it himself."

Speaking to CNN, Stone scoffed: "I called on Trump supporters to go to their delegates, find them at their hotel, and ask them to sign a [pledge] to respect the will of the voters. I'm not for violence."

As for Cruz's preoccupation with him, Stone says, "It's obviously some kind of obsession. He must lay awake thinking about what I am doing."

The Cruz campaign declined to comment.

The big question on the mind of Cruz and the Republican establishment is just what kind of trouble Stone will get into at the GOP convention.

Stone says he's free to do whatever he wants, since he's not bound by any formal role in the Trump campaign organization.

"I'm my own person," Stone said, "I don't have to get clearance for things I want to say."

"I'm going as an FOT," he added. "Friend of Trump."