When David Stockman was testifying last week on the new Reagan budget, an extraordinary thing happened. Delaware's bright, blunt Democratic senator, Joe Biden, threw partisanship to the winds in an election year. He said it would be hypocritical for Democrats not to praise Mr. Stockman and the Reagan administration for getting inflation under control and restoring public confidence. He said it would be equally hypocritical for Republicans not to admit that coming megadeficits amounted to ''mortgaging our future.''
He then proposed that even in this partisan year, both sides get together out of the public eye and work out a solution to curb the superdeficits.
Biden is not the only Democrat in town willing to forgo the opportunity of rubbing the deficit salt into the elephant's hide. Sen. Lloyd Bentsen, a Democratic tax and debt specialist, is calmly laying a course toward major tax reform to quell the deficits. House Budget chairman Jim Jones sees eye to eye with GOP pragmatists like Senate Budget chairman Pete Domenici on the general direction in which all sides must move.
But the big questions remain. What kind of action will be taken? And how soon?
If the Biden-Domenici-Stockman spirit prevails, American society may win long-range benefits from this brush with economic disaster - benefits that tax reformers had almost despaired of ever seeing arrive.
The common wisdom is that nothing can be done but to talk quietly about deficits and wait for the election to pass. There is no reason the American voter has to accept that timetable. There is, in fact, a risk in accepting it: the risk that after the election it may seem easier to patch together a temporary surtax rather than make a lasting change in America's national economic system.
If we are fortunate and the pragmatists of both parties prevail, something like this sequence may occur:
* President Reagan's proposed bipartisan commission on deficits will provide the right atmosphere within which bipartisan congressional budget leaders can quietly work on a package this year.
* Even if that package is not delivered before the election, it can be ready for delivery.
* Instead of an income surtax, the keystone of the deficit-reduction package could well be a consumption tax (favored by Bentsen) or a modified flat tax (favored by Biden and New Jersey's Bill Bradley, as well as many Republicans).
A consumption tax would probably not take the form of the politically risky value-added tax (which taxes each stage of the manufacture of a product and rebates taxes on exported goods to encourage trade). A realistic consumption tax is more likely to resemble a national sales tax, with exemptions for food, clothing, and health care. The modified flat tax would create a simple tax form with no (or few) deductions and perhaps only three brackets.
Surely it is politically naive to suggest that so fundamental a tax reform could be accomplished so quickly. Especially since most tax-reform attempts in recent decades have ricocheted in the direction of more loopholes instead of more simplicity.
Senator Biden's impassioned answer, which Senator Domenici seconds, is that ''literally the future's slipping from our grasp.'' And that threat may at last be the stimulus for changing the American economy to one in which consumption is moderated, saving rewarded, and future generations are not mortgaged to cushion the present.
Furthermore, says Biden, ''This President (Reagan) is the one president in my lifetime who could pull it off and make it a plus.''
Why can't we put off this economic change until next year?
1. Because neither White House nor newly elected congressmen and women will feel as much pressure from voters then.
2. Because some of the hyperexpensive weapons systems now in the drawing board stage will meanwhile pass the cutoff point.
3. Because the nation will have moved much closer to the brink. In just 31/2 years' time, if nothing is done to change the current deficit course, interest on the national debt could rise as much as 65 percent.
Shakespeare was not, fortunately for his reputation, a political columnist. But he turned a phrase four centuries ago that fits the current situation better than designer jeans. Seldom has there been a better example of ''a tide in the affairs of men, Which, taken at the flood, leads on to fortune (or at least away from misfortune).''
That tide is public awareness, even massive impatience, over megadeficits. It's a tide that's running now, not waiting for 1985.