Sen. John McCain’s limited release of his medical records last spring had on-again/off-again drama that’s now a hallmark of his campaign. It also left the world with insufficient data by which to judge McCain’s chances of surviving a term or two as president.

Once McCain tapped Alaska Gov. Sarah Palin as his running mate, the summer’s steady stream of speculation on his life expectancy began to rise rapidly, flowing from all quarters and corners of the globe — from baseball stat-heads to L.A. psychics to actuaries in Australia.

It was, coincidentally, revelations in medical records that led Sen. George McGovern to drop Sen. Thomas Eagleton as his running mate on the 1972 Democratic ticket — a move McCain has increasingly been called upon to repeat over the course of Sarah Palin’s agonizing, slow-drip roll-out as vice presidential nominee (which has already outlasted Eagleton’s 18 days). A poor showing at Thursday’s vice presidential debate could release the floodgates.

Waiting in the wings would be veepstakes runner-up Tim Pawlenty, who last weekend winged to Europe for an abruptly announced trade mission studded with high-level diplomatic sessions — a well-timed occasion for Minnesota’s governor to burnish foreign policy credentials conspicuously missing in his Alaskan counterpart.

Should McCain seek to switch sidekicks at the moment that United States is toying with global economic collapse, the already-vetted Pawlenty — who keynoted an international conference in Madrid on Monday and takes tea at 10 Downing Street today — would bring to the fading GOP ticket fresh and timely lessons from Spain and Britain on how best to slip gently into the role of a fallen world power.