The Reagan administration, in the face of a challenge from Walter Mondale, is keeping up the pretense that the huge national budget deficit is an unimportant fleck on the horizon, to be swept away by economic boom.

Yet, at the same time, the White House has sanctioned an all-out drive by Treasury Secretary Donald T. Regan to devise a series of gimmicks that might lower the interest being paid on the debt, or that might have a cosmetic effect on the deficit in the next few years by postponing interest payments.

A major step, of questionable morality, is intended to induce foreigners to lend more money to Uncle Sam, by enabling them to purchase Treasury bonds -- or private issues backed by Treasury bonds -- without revealing their names. The theory is that foreigners, ifprovided with a cozy and anonymous tax shelter, will be willing to accept a slightly lower rate of interest.

It is a Faustian bargain, and a bad one, to which this government is driven by the hugeness of the deficit and the need to find help in financing it with money from abroad.

Gimmicky ways of meeting the debt problem have been attacked by key figures of both parties on Capitol Hill, notably Republican Sens. Robert Dole of Kansas and Dave Durenberger of Minnesota, and Democrat Howard Metzenbaum of Ohio.

The Senate also passed a resolution by unanimous voice vote condemning the issuance of bearer, or unregistered, securities. Yet, the Treasury, says Metzenbaum, has flagrantly ignored the will of Congress by continuing to sanction the sale of private- bearer securities, backed by Treasury paper, to foreigners.

"I resent what the secretary of Treasury is doing," Metzenbaum said in an interview. "First, he sponsored elimination of the 30 percent withholding tax (formerly paid by foreigners on Treasury interest they earned), and now he's compounding the problem by making it possible for (private) institutions to issue bearer bonds.

"It's upsetting to see that Regan, who should be on our side, has become the enemy."

What Metzenbaum and others fear is that Swiss banks -- to take one example -- will be able to invest huge sums in bearer bonds backed by Treasury securities, and assert that none will be resold to American citizens. But since the Swiss banking system maintains complete secrecy, there is no way of ensuring that only foreigners will own the bonds -- on which they pay no taxes.

Regan insists that the Treasury has its own ways of enforcing compliance, but many on Capitol Hill doubt it.

The Treasury also seriously considered -- until it encountered what it said were technical problems -- the issuance of "zero-coupon bonds." These bonds would delay the payment of interest by, say, 20 or 30 years. Thus, succeeding generations, not this one, would pay for the cost of borrowing now, while the annual interest burden over the next several years would look smaller.

Regan has put this particular gimmick on the shelf for now, but will make it easier for private-bond companies to repackage Treasury securities into zero-coupon form for sale domestically.

The bearer-bond issue ought to be resolved quickly. It seems plain wrong to sell bonds to foreigners who not only pay no taxes, now that the 30 percent withholding tax on the interest earned has been scrapped, but can keep their identity a secret. This compounds the potential for tax abuse and evasion. Not only is the Treasury plan, in effect, a sleazy partnership with tax evaders, but it belies the presumed intention of the Reagan administration to reduce the size and cut the costs of government.

The Wall Street Journal reports that the Treasury decisions have touched off a boom in the American bank andbond-selling business in Europe. The loophole that used to exist through sale of bonds issued through a tax haven, the Netherland Antilles, is now to be legalized right out of New York. It could, some say, be a $50 billion annual business, and everybody wants a piece of the action.

Metzenbaum plans to seek some legislative remedy, either a separate bill or a rider to some measure that the president must sign, to keep the Treasury on the straight and narrow. The fact that the earlier resolution passed with no objection from Finance Committee Chairman Dole indicates wide bipartisan support.

Unless Congress asserts control, nobody knows where the Treasury's "creative" financing will end. But one thing is clear: the more Treasury borrowing that is made tax-exempt for foreigners, the more American citizens will have to foot all the bills.