With polls indicating that of post-World War II vice presidents he is the least credible to become president, Dan Quayle has long been a political disaster waiting to happen.

Now President Bush's medical problems have focused the spotlight.It's not just a question of weak Democrats' starting to smell some faint 1992 opportunity or of unlikely scenarios for a Republican civil war over replacing Quayle.

There's obviously a chance, if things go awry, for his very selection to be an embarrassment to the man who picked him.

Since Quayle became vice president in 1989 the percentage of Americans comfortable with his becoming president has dropped from roughly a third to just 19 percent.

Having no national image when Bush picked him, Quayle has no bolstering prior stature - and worse, no ability to reshape a lightweight persona molded by his ignominious 1988 debate with Lloyd Bentsen and two years of Jay Leno and Johnny Carson jokes.

Right after the 1988 vote, Republican strategists estimated that Quayle cost Bush 1 to 3 percentage points. The lower number was probably more accurate, but now, given the fear of Quayle that was many Americans' first reaction to the news of the 67-year-old president's heart problems, it's possible the drag could be 2 or more points.

If Bush's fibrillations recur, the albatross factor could soar to 3 or more points.

That's not a problem for Bush now. He can afford a drag, even though Republican 1992 hopes of a 61 percent blowout have cooled as Americans have stopped calling the gulf outcome a victory with Saddam Hussein still in power.

With the Democrats in disarray, Bush still appears headed toward a 55 percent to 58 percent landslide.

The caution is that surprise economic problems, the Middle East and even the claims of the 1980 Iran link could drop GOP prospects into the 52 percent to 54 percent range.

Then, especially if Bush's ailments remain, a 2 or 3 point Quayle drag could be an expensive beau geste.

Bush certainly doesn't want to drop Quayle, but any such decision would have to be made by fall or winter.

Moreover, there are countervailing arguments.

Signs that Quayle could be dropped might open a Pandora's box of ambitions among Republicans - from James Baker and Jack Kemp to Sen. Phil Gramm of Texas.

Conservatives might also be tempted to mount a primary challenge against Bush.

Finally, if the president's health intensifies the spotlight on Quayle it will also draw attention to why Bush picked him, an enigma that still perplexes political veterans.

A belief at the time that he would attract the youth or female vote is an unindictable miscalculation.

The possibility that Bush wanted a harmless acolyte, not a rival from his own generation, may be weak leadership, but it is hardly damning.

As for the half-joke about Quayle's being term insurance, it's not taken too seriously because it's too unnerving to think about.