It’s easy to figure why so many people want to be president. The fame. The historical stature. The fortune to be made afterward on the speaking circuit.

After more than a year of mud, maulings and mischief on the campaign trail, it’s less obvious why the candidates persevere.

Is the quest worth the tar that can last a lifetime? The smears that wound precious family members?

Yet the answer is a surprising “yes” – at least for leading Democrats Hillary Clinton and Barack Obama. The reasons have less to do with the tarnish than with the opportunity to show character and leadership.

Clinton started out as Mount Everest, confident and unassailable. For all her negatives among bedrock conservatives, she seemed the pinnacle of the kind of smart, let’s-make-a-deal politics and campaign seasoning that Democratic Party insiders figured they would need in November.

That was until hubris, her lies about dodging sniper fire in Bosnia and a failure to mask her campaign’s attacks against Obama impeded the juggernaut. Things tumbled so low that voters even started asking Chelsea about Monica. Before her 9-point Pennsylvania win Tuesday, Clinton’s smile looked pasted in place as Pittsburgh Steelers owner Dan Rooney sidled into the Obama camp.

Obama also was buoyant when his journey started. He was JFK and Reagan rolled into one, a man who’d lived the American dream, who had personally bridged the racial divide, whose inspiring words were bringing record numbers of young and disillusioned voters to the polls.

But in Pennsylvania, weighted by the negatives, he came across as preoccupied, stumbling over words, visibly uncomfortable in the final harsh pummeling that was the Philadelphia debate, trying to look upbeat, and coming across just as dog-tired.

“How Obama Fell to Earth,” is the way New York Times columnist David Brooks framed this revelation that the candidate was human.

Well, thank God the candidates ARE human.

Voters tend to gain more from seeing beneath the stock smiles and canned talking points than they would from a hundred issue statements.

The public – and media – preoccupation with the tawdry, the mangled and the scandalous is not something necessarily to lament; it can be something to celebrate.

Negatives test the candidates’ mettle.

Unexpected challenges and revelations are what expose weaknesses – or strengths – in a prospective commander-in-chief. Instead of pulling a candidate down, they can reveal the real depth of leadership, knowledge, poise, personality and understanding – all qualities highly relevant for the demanding job of president.

America has been on top for so long that, a decade ago, its voters might have been excused for thinking it didn’t matter so much who was president. Few voters cared when candidate Bush seemed largely indifferent to what was happening in Europe or Asia, and ignorant of the names of some key foreign leaders. And so what if he might even have been AWOL for stateside military service as he ducked Vietnam duty?

Now we know it does matter. It matters very much. After 9/11, after Iraq, Americans want a smart, engaged, intense doer as president, not just a pal.

The negatives are that test. And so far, in a crazy, roundabout way, the Democratic contenders are passing those tests.

It’s the negatives that enable Clinton to say she’s tough. No setback or trough has been too low for her to believe that her political instincts, tirelessness and intelligence couldn’t pull her through.

It’s the negatives that help Obama set his credentials for November, against the notion that he’s too politically raw and untested to be president.

And as Obama himself said in surprisingly upbeat comments on “The Daily Show with Jon Stewart,” one of the most affirming goods of being out on the campaign trail is that – even as you get pummeled – you also have the chance to really drink in what’s bothering America. It turns out that folks from the Great Plains cornfields to manufacturing ghost towns of the Midwest to the Ivy Leagued coasts care about remarkably similar issues.

Too much slime may be bad for the nation’s psyche. Yet if some of that mud helps reveal to us what we really need to know about our next Decider, it may well be that we’ll all be better off.

Elizabeth Sullivan is foreign affairs columnist and an associate editor of the editorial pages for The Plain Dealer of Cleveland. E-mail [email protected].