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No-one in America knows more about the political dark arts than Roger Stone. His friends would say he can deal with any dirty trick; his enemies would reply that he invented most of them.

As perhaps Donald Trump's closest political adviser, he was accused by Senator Ted Cruz during the primary campaign of having half a century of trickery behind him, and being the master of the smear. Mr Stone responded by calling the senator a liar and comparing him to Richard Nixon, always a low blow.

And he should know. He learned his trade as a political fixer in the Nixon White House, where, he's happy to admit, he was part of the rough political generation of the Watergate era. He is smooth, soft spoken and funny, and relaxed in a white suit and the kind of brown and white shoes that a Gatsby might have worn. But he's as tough as they come.

We spoke in Cleveland about Mr Trump, his friend for nearly 40 years. He acknowledges the candidate's penchant for shooting from the hip, his egocentricity, and his unpredictability. So why was he fit to be president? "He's smart - don't forget that - and he listens, probably more than you think."

Image copyright Getty Images Image caption Roger Stone (r) was an advisor to President Ronald Reagan

I asked him how we might picture him, as president, sitting down in Russian President Vladimir Putin's office. "Putin will respect him more than he respects Obama. I'll tell you that. They have a lot in common."

Then, we talk about the Republican Party. The party of the Bushes - no-one from that clan has come to this convention - is, to Mr Stone, a thing of the past that is gone for ever. "Trump is the outsider who has changed everything. This is like nothing we have seen before. The times have changed completely."

He's too long in the tooth to predict the course of the campaign - which he thinks lies 50/50 as things stand - but he will admit something that the grizzled veterans of the circuit know for a fact but sometimes try to put out of their minds: this will be the "dirtiest, most bruising" campaign of them all.

So I asked whether he, and the others who brought us campaigns dominated by attack ads, felt regret. Hadn't the public discourse been poisoned by all this? Not at all, says Mr Stone. Remember that when Abraham Lincoln was running for election in 1860, there was a leaflet campaign against him in Illinois based on the rumour that he'd fathered a mixed-race child.

Image copyright Getty Images Image caption Roger Stone honed his skills in the Nixon White House

Take from that what you will. It's clear that this will be a brutal piece of hand-to-hand combat. The Republican message, hammered out every evening at the convention this week like a drum beat, is that Hillary Clinton - the secretary of state who set up her own email server - should be made to pay. "Hillary - prison 2016," say the placards.

Mr Stone hardly needs to be tempted to launch into vitriolic attacks on the Clintons, for whom he has a special loathing. Their charitable foundation, for example, is to him "organised crime".

This is politics in the raw. And with Mr Stone at the heart of the Trump campaign, it's obvious that although the candidate's tone may change a a little - Mr Stone reminds me never to forget that Mr Trump has never done this before, at any level - he will end as he began, as a street-fighter.

Image copyright Getty Images Image caption Mr Stone believes Donald Trump's unorthodox style will scupper Hillary Clinton's campaign

We'll see that, especially, in the three debates scheduled for later September and October. But I put it to Mr Stone that Mrs Clinton is a lawyer with a razor-sharp mind who will be ready for anything Mr Trump throws at her.

His response is that she'll find it hard to deal with someone who is as happy to break the rules as Mr Trump. "He'll say things that no-one else would say." And he makes a surprising observation about the woman for whom so many Republicans have developed a dislike that sometimes feels like an obsession.

"I think she has a glass jaw." In the boxing ring - it's Mr Trump's favourite sport - that's fatal for a fighter. Mr Stone thinks she will find it hard to match the most unorthodox bruiser she has ever had to tangle with.

On the other side, they disagree. For the Clinton team, the debates are a happy prospect. They think it will be Mr Trump who ends up on the floor.

We shall see. But with Roger Stone in his corner, we can say one thing. This campaign, noisy and even raucous as it has already become, is going to be bloody. For the man who will prepare Mr Trump for the ring, there's no other way.