Back when men still wore hats and ladies wore gloves, my precinct committeewoman made me memorize an iron rule of American politics: A week is a lifetime politically, and three months is an eternity.

That wise maxim has apparently been forgotten by the would-be 2012 Republican challengers to President Barack Obama, not a single one of whom — with quite possibly only 40 weeks remaining before the first, real main event of the 2012 presidential nominating fight, the crucial Iowa caucuses — has yet to declare her or his White House candidacy.

Because our presidential elections are held on November in years divisible by four and because organizing separate presidential organizations simultaneously in several dozen states requires people, money and, most of all, time, a candidate must generally decide to run no later than late winter of the year before the election. That would be now.

The problem is that the decision to challenge or not to challenge an incumbent president is almost always overly influenced by the challenger’s judgment of the incumbent president’s political strength a year and a half before the actual general election voting.

Consider the late winter of 1983, when President Ronald Reagan’s re-election prospects for November 1984 were, to put it bluntly, bleak. When asked by the Gallup Poll, “Do you approve or disapprove of the way Ronald Reagan is handling his job as president,” only 35 percent of the Gipper’s constituents gave him a positive grade.

So what happened? Democrats rushed into the race — including Walter Mondale, John Glenn, Gary Hart, Alan Cranston, George McGovern and Jesse Jackson — sure that Reagan was beatable. But if a week is a lifetime politically, then a year and a half is a millennium. By November 1984, Reagan was getting favorable grades from 60 percent in the Gallup Poll and, more important, was carrying 49 of the 50 states while wining a landslide re-election.

In 1988, Reagan’s vice president, George H.W. Bush, won his own White House race, and as he prepared in 1991 to run for re-election, he looked unbeatable. After Saddam Hussein had invaded and occupied Kuwait, Bush 41 and his secretary of state, Jim Baker, organized a 32-country coalition to drive Saddam from Kuwait, won support for that action from both a Democratic Congress and from the United Nations and then, in a remarkable four-day military offensive, won the first clear-cut American military victory since 1945.

Bush in the winter of 1991 — a year and a half before his re-election day — stood at an unprecedented 89 percent favorable in the Gallup Poll. One by one, leading Democrats — Dick Gephardt, Al Gore, Lloyd Bentsen, Bill Bradley and Mario Cuomo — who had openly or secretly lusted after the presidency saw Bush as unbeatable and came to the same decision, some variation of: “Rather than run, I choose to spend more time with my family.”

Only five Democrats rolled the dice that year — Tom Harkin, Bob Kerrey, Jerry Brown, the late Paul Tsongas and Bill Clinton. By Labor Day of 1992, Bush’s favorable number in the Gallup had plummeted to 29 percent, and he would lose that November to Clinton. A week is a lifetime.

As of today, Obama looks formidable: a 50 percent favorable rating according to Gallup, no apparent primary challenge and, quite possibly, the nation’s first $1 billion campaign treasury. But Republicans should remember 1984 and 1992 before concluding that the race against Obama is impossible. If gasoline goes to six bucks a gallon — not impossible — no incumbent will be safe in 2012. Because a week truly can be a lifetime in politics.

Mark Shields is one of the most widely recognized political commentators in the United States. The former Washington Post editorial columnist appears regularly on CNN, on public television and on radio. Click here to contact him.