Johann Hari: Scribbled in the margins: the character of the next President

The Presidential debates are ending with Barack Obama roller-coastering up the opinion polls as the old economic scenery collapses behind him. Here, in the middle of 1929, it's tempting to write this off as a race between the first black man to run for President and the first corpse but this isn't over. Even now, John McCain is poking his head out of the rubble only six points behind. The insipid soundbites of a presidential campaign give little guide to how a candidate will govern: remember George W Bush promising a "more humble foreign policy" at every debate in 2000?

The clues to the real man are often scribbled in the margins. Perhaps the best hint is in the thinkers both candidates have named who have most shaped their political thought. Their reasons are revealing – and shocking.

John McCain chose Theodore Roosevelt, the 26th President of the United States. This isn't pick-a-name boilerplate: in two books, McCain has written long adoring chapters saying he is the most powerful influence on his life "except for the navy". So in the story of this past president, we may find the gut instincts of the next. "Teddy" Roosevelt was a feeble, sickly child born in 1858 into American aristocracy, a family of New York bankers and military men. He became obsessed in childhood with Strength and Willpower, determined to prove that, despite his failing lungs and ailing chest, he was A Man. He began to obsessively exercise and read war stories. He worked his way up eventually landing up as an improbable vice-president, elevated to the Oval Office by a bullet.

All his life, Roosevelt saw war as spiritually uplifting, a way of "toughening" American men and allowing them to "prove themselves". In 1897, he wrote: "I should welcome almost any war, for I think this country needs one." McCain cites this approvingly, sighing: "Roosevelt believed fighting was essential to a happy life. I know what he meant." So Roosevelt set out to trick and provoke the US into a series of invasions. As assistant secretary of the navy, Roosevelt personally commandeered a cavalcade of men to charge into Cuba, drive out the Spanish and claim the island. McCain writes approvingly: "He surveyed the awful carnage, the torn earth, the trenches filled with enemy dead, and pronounced himself delighted with the day."

He then lobbied hard for the US to seize the Philippines from the retreating Spanish. The islands were resource rich, and stood at the golden gateway to Asia. But the Filipinos wanted to rule themselves – so Roosevelt called for them to be crushed. The leading US general was asked what age they should start massacring Filipinos, and he said: "Anything over 10," adding: "I want no prisoners. I wish you to kill and burn: the more you kill and burn, the better it will please me." Three million people – overwhelmingly civilians – were slaughtered by soldiers who included McCain's grandfather. Roosevelt called it "the most glorious war in our nation's history".

And he went even further. He wanted to take over Panama and build a canal for US goods through it. The Colombian government refused to surrender its territory. So he spent a fortune trying to stir rebellion in Panama, and as soon as there was a hint of it, he sent in the US navy to grab the land. McCain calls this "energetic and forward-looking". The Attorney General called it "illegal". McCain says now that Roosevelt's foreign policy is his model. "He sought to preserve peace and order," he claims, "by confronting potential adversaries with America's resolve and readiness to fight, if necessary, to protect its interests."

There was a more appealing domestic side to Roosevelt. He was disgusted by corruption the moment he encountered it in the New York Assembly, and spent much of his career crushing it. McCain reacted to discovering corruption in Congress by gobbling it up. He took money from the fraudster Charles Keating and in return lobbied to stop the government from investigating him. Only when this was exposed did McCain "discover" Roosevelt's reformism, and demand a small dose of it. It is the aggression that he loves in his Teddy, not the reform.

Barack Obama's intellectual hero is a less well-remembered man – and seems, at first glance, a strange choice. The apostle of Yes-We-Can has picked Reinhold Niebuhr, a downbeat, Protestant theologian who referred to his co-believers as "the children of darkness". But as you probe Niebuhr's thought, you see that his contradictions and complexities intriguingly mirror Obama's.

Niebuhr was born to German immigrant parents in 1892, and became a church minister. He made his name in the 1920s defending the local workers for the Ford Motor Company against gross exploitation, and standing up to the Ku Klux Klan. His political thought veered wildly: he shifted from pacifism to advocating violent revolution to becoming an establishment Cold Warrior given medals by Lyndon Johnson. He has been dubbed "A Man For All Reasons", quotable for any cause, any time.

But there was one consistent core to his thought. Niebuhr believed that all human beings were fallen, and "even the best men and nations" are merely "compounds of good and evil". Even when you believe you are doing good, you can do evil. Nobody has "a halo of moral sanctity"; nobody is innocent. Obama says he took from Niebuhr his conviction that "there's serious evil in the world, and hardship... and we should be humble and modest in our belief we can eliminate those things". Where Roosevelt saw history as a long martial victory parade, Niebuhr saw it as a drive through darkness in a creaky vehicle.

Here, then, is the best clue to Obama's presidency. He is drawn to Niebuhr's later writings, when he offered a theology of working within the system, and trying to nudge it very slowly leftwards. He said that to be "realistic", you have to understand "the structure of nations and empires", and squeeze your modest hopes of improvement into that framework. Noam Chomsky called Niebuhr "the theologian of the establishment", because "he presented a framework which, essentially, justified just about anything they wanted to do". There's some truth in this: Niebuhr offers a recipe for compromise with the unacceptable. It is centrism with a guilty conscience.

Imagine a presidential debate between Theodore Roosevelt and Reinhold Niebuhr. It would look remarkably like McCain vs Obama but with the underlying philosophies laid bare. So does the US want as president a war-hungry aggressor who believes in the USA's right to do anything – or a self-doubting centrist, aware of his country's flaws but willing to correct them only in part, and with sceptical slowness?

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