Why is the Republican presidential field so weak?

Six months ago, that might have seemed an unfair question, or at least premature. The roster of candidates often starts out looking like the “seven dwarfs,” only to have some rise in stature while others fall away.

That hasn’t happened this time. Mitt Romney looks no less presidential today than he did at the start. But none of the others has come close to making himself plausible.

Ron Paul, second-place finisher in New Hampshire, remains what he always was: a movement leader, an advocate for both attractive and highly unattractive tenets of libertarianism — a fringe candidate.

Rick Santorum, launching from the unlikely platform of a losing Senate race, has come across as a sincere but sour, less inclusive, smaller-bore version of Mike Huckabee. Rick Perry, owner of the most promising resume, opened by calling the Federal Reserve chairman a traitor and went downhill from there. Newt Gingrich has demonstrated a disqualifying ego, which takes some doing in this business. Jon Huntsman has demonstrated that he can speak Chinese.

And already we are doubting our memories: Were Herman Cain and Michele Bachmann really serious candidates?

One of these people might have surprised in the Oval Office. Science has yet to discover how to predict which Kansas City haberdasher will exceed low expectations and which Georgia peanut farmer will fulfill them. It’s also true that the fantasy candidates who didn’t declare — Paul Ryan, Mitch Daniels, Chris Christie, Marco Rubio, Jeb Bush, David Petraeus — would, under the scrutiny of press and rivals, have turned out to be human, too.

Still, on all available evidence it was and remains a weak field. So, again: Why?

It could be the luck of the draw. Every first-grade teacher will tell you that some years are better than others.

It could be that more serious presidential candidates, sizing up the incumbent in 2009, when serious campaigns had to begin, decided, not illogically, that President Obama was likely to win. Let someone else be the John Kerry of the Republican Party. Come back in 2016.

It could be that the process has devolved, for some, from daunting to repellent: the number of millionaires whose egos must be stroked on the way to raising $1 billion, the smears from unaccountable political action committees, the dwindling media interest in substance, the Twitter-paced cycle that makes the Clinton war room look like something from the vacuum-tube era — it may be a quadrennial bar to many people of quality.

It could be that serious people looked at the decisions that will have to be made in the next four years and concluded that the job would not be much fun. Taking charge in an era of rising health-care costs and an aging population doesn’t seem, at first blush, a road to popularity.

But in another year, that challenge might have motivated top-flight people. After all, the country’s travails offer an opportunity for fundamental reform that a true leader would jump at — to reshape the tax code, say, to encourage things we like (working and saving) and discourage things we don’t (burning oil, gas and coal). Such big things could be done, for political and substantive reasons, only in a bipartisan fashion.

For their own reasons, Obama and the Democrats haven’t seized that opportunity. But why have visionary Republicans shied away?

The nearly forgotten candidacy of Tim Pawlenty offers a clue. Once upon a time a conservative governor from a swing region with a record of working across the aisle might have gained traction.

But in a party that has come to loathe compromise, Pawlenty didn’t have the gumption to run on his record, and he couldn’t sell himself as less nice and more ideologically pure than he really was. When he couldn’t bring himself to be mean to Romney in an early New Hampshire debate, he was finished.

The Republican ideology of no new taxes, ever, is a straitjacket. But even more dispositive is the conviction that reaching across the aisle is weak and treasonous.

Until that conviction fades, politicians who want to get things done, and would know how to strike deals in the nation’s interest, may stay on the sidelines.