Washington ? The conventional wisdom is that Barack Obama cannot be beaten. The root of this wisdom is the aphorism, sometimes attributed to former New York Gov. Benjamin B. Odell Jr. and sometimes to former House Speaker Joe Cannon, that you can’t beat somebody (Obama) with nobody (any one of the dozen Republican nobodies, male and female, Trump and trumped).

The provenance of that aphorism, which puts it at the beginning of the 20th century, points to the fallacy of that aphorism. Since then, nobodies, or near-nobodies, have done fairly well. Five have been elected president since that time — in 1920, 1960, 1976, 2000 and 2008.

Partisans of those five will howl in outrage at that characterization, but were Warren G. Harding, John F. Kennedy, Jimmy Carter, George W. Bush and Barack Obama substantially more established political figures the year before they were inaugurated than the current crowd of Republican possibles?

Sen. Kennedy and Sen. Obama — both charismatic and eloquent campaigners — scored historic victories when they became the first Catholic and black presidents, respectively. But neither was an inevitable nominee, let alone a favorite, for the White House at the time the 1960 and 2008 campaigns began.

GOP has some howlers

There are, to be sure, some howlers in today’s Republican field. But is Rep. Michele Bachmann, the tea party firebrand from Minnesota, more or less outside the American political mainstream than, say, Rep. Dennis Kucinich, who has run for president twice in Democratic primaries? Bachmann is part of a broader political movement that helped elect substantial numbers of House members last year and is an Iowa native, no small advantage.

Former House Speaker Newt Gingrich of Georgia, whose prospects grow dimmer by the day as details of his personal life are examined, is only slightly less a has-been than was Sen. Mike Gravel of Alaska, a Democratic candidate in 2008. Both are historical relics; one brought to an end 40 years of Democratic rule in the House, and the other is remembered for placing the Pentagon Papers into the Congressional Record.

Nomination boosts stature

But the important thing to recall is that presidential challengers almost always seem weaker until they get the nomination, when their influence and appeal grows. The very act of accepting a major party presidential nomination has the effect of one side of the mushroom in “Alice’s Adventures in Wonderland,” which made her grow very tall indeed.

Indeed, the Alice Effect transformed Kennedy from a senator taking on a sitting vice president into a political powerhouse who, at his nominating convention in Los Angeles, spoke of a New Frontier. Consider the speech he delivered there in the Los Angeles Coliseum:

“The New Frontier is here whether we seek it or not. Beyond that frontier are uncharted areas of science and space, unsolved problems of peace and war, unconquered problems of ignorance and prejudice, unanswered questions of poverty and surplus. It would be easier to shrink from that new frontier, to look to the safe mediocrity of the past, to be lulled by good intentions and high rhetoric …”

Had that speech been delivered by a junior senator on the Senate floor, it would have been remembered by nobody, except perhaps Theodore Sorensen, who helped write it. But today it is remembered as a signature speech of the era, and the phrase is indelibly imprinted on the American character. A presidential nomination has that effect, and whoever heads the GOP ticket next August will have that platform — and that stature.

That’s why Obama’s re-election is not assured, despite the apparent weakness of the GOP field. That said, Obama has many advantages.

First is the presidency, of course, which confers upon him a gravitas and glamour that no challenger can match. Then there is his robust fund-raising operation, which grows out of his residency in the White House and his efficiency in raising money. That was on full display during his 2008 campaign, and allowed him to conduct $35,800-a-plate dinners like the one he held at the home of Jon Corzine, a former senator, governor and Goldman Sachs chieftain.

Obama also has the power to control the political agenda, though in recent weeks he has ceded that to the Republicans, who have made the deficit the defining issue of the time, drowning out the surprising notion that low interest rates have rendered the cost of serving the nation’s ever-bigger debt the lowest it has been in more than a dozen years. Still, the deficit remains a huge problem — and a huge drag on the Obama re-election campaign.

Unpalatable debt choices

Nobody wants to deal with the debt right now. Not the president, because the choices are politically unpalatable to Democratic interest groups, and not the Republicans, because the longer the issue persists the better are their prospects in 2012. Otherwise the budget question could be resolved in 25 minutes of reasonable compromise involving the Social Security retirement age and tax caps, Medicare benefit levels and eligibility ages, military spending cuts, and a comprehensive overhaul of the income-tax system that would please both the left (by eliminating loopholes) and the right (by lowering rates).

As St. Augustine would say if he were a member of the House: Give me budget discipline, but not yet.

The Republicans also seem to be saying: Give me a 2012 frontrunner, but not yet. But they’ll have one soon enough, and when the eventual nominee walks onto the stage at the first debate next year, he or she will have the same podium and the same opportunity to score points as Obama.

Of all the ladders of social mobility in America, none is steeper than a presidential nomination. It allows a nobody to become a nominee and thus a somebody in an instant’s time. The person who knows that better than anyone on Earth is … Barack Obama.