George W. Bush was not an enigma. He had no hidden parts.
His father was not mysterious. George H.W. Bush's life was dedicated to achievement and service.
Even Bill Clinton wasn't unfathomable. Nothing in his presidency -- the brilliant highs, the shocking lows -- was a substantial, unpredictable departure from his past.
Barack Obama, though, is the most enigmatic president since Jimmy Carter, the most mysterious since Lyndon Johnson, the most unfathomable since Franklin Roosevelt.
Political professionals sometimes say of public figures that what you see is what you get, more or less. But with Mr. Obama, what you see is both more and less than what you get.
All this is on display as Mr. Obama runs for president in the same economic crisis that helped catapult him to the White House four years ago. His first term has been disappointing; even he implicitly acknowledges that.
He looks to renew his vows with the American people -- the 18th-century English pundit Samuel Johnson would call that the triumph of hope over experience, his classic definition of second marriage -- and he's returned to his most comfortable role: candidate.
A third of a century ago, American pollsters and consultants began speaking of a "permanent campaign" -- the notion originated with Carter pollster Patrick Caddell -- that transformed the act of governing in the White House into an extension of campaigning for the White House.
But there was an abrupt change between Mr. Obama's campaign, which seemed so beguiling, and Mr. Obama's presidency, which managed to repel his allies on the left even as it consolidated, even fortified, his opposition on the right.
Mr. Obama was a silver-tongued orator in the campaign, but he lacked a silver bullet in the presidency. He was a darling on the stump, a dud in office. This is not a remarkable view. It is held in the White House itself.
Part of the reason was the hand he was dealt. No one underestimates the rot in the U.S. economy, made worse by the crisis in Europe that Mr. Obama cannot be expected to control and the competitive challenges from Asia that former Mitt Romney's proposals also would only glancingly affect. But no one assumes the presidency without anticipating difficulty and unpredictability.
Mr. Bush the younger understood this. When a White House visitor expressed sympathy for the hardship he faced after the 2001 terrorism attacks, he said that handling such challenges was precisely why he sought the office.
So it was, presumably, with Mr. Obama. He ran for president to deal with the economy, not to be burdened by it, and to change the way Washington worked, not to bemoan it.
Outside the Washington Beltway, and perhaps inside it as well, the President seems to be two men, one a brilliant practitioner of the political arts, the other a conscientious objector to politics.
But politics comes in two dimensions. A skilled president must know how to get the office and then know how to use it. Failed presidents triumph in the former and stumble in the latter.
The gravest warning sign in Mr. Obama's background wasn't his spare record in the U.S. Senate, nor his limited experience in elected office.
Instead, the most troubling aspect of Mr. Obama's past were the 129 abstentions in his Illinois Senate career. They suggested that Mr. Obama was more interested in getting elected than in doing the work he had been elected to perform.
Few accuse President Obama of being a shirker. In any case, no one measures long-term impact by the length of a president's day or his attention to detail -- not since Ronald Reagan and Mr. Carter.
But the mystery about this president is why he has not been able to match his poetic style of campaigning across the country with the prosaic business of governing the country.
In 2008, Mr. Obama was a phenomenon as much as a candidate. He sowed excitement not seen since John Kennedy, and promised a change in governing approach not seen since Mr. Reagan.
Now he is campaigning again, this time lacing his effort with blistering critiques of Mr. Romney, many of which seem to have damaged his rival.
But the election in November is far less about Mr. Romney than it is about Mr. Obama. It is also about this stark fact: This is the first election since 1992 in which an incumbent president is in the position of asking not only for a second term, but also for a second chance.
David Shribman is executive editor of the Pittsburgh Post-Gazette.
Contact him at: firstname.lastname@example.org