In a historic white brick townhouse on Jackson Place--Teddy Roosevelt once used it as a temporary White House--the small professional staff of the Presidential Commission on Social Security Reform is hard at work.

The commission, under the chairmanship of Alan Greenspan, former chief economic adviser to President Ford, faces an enormous task. By the end of this year, it must come up with recommendations that will save the Social Security system from going broke.

There may be no more important economic-political-social problem than this one: Social Security is the main source of income for three out of every five elderly people in this country. But inflation and recession threaten the solvency of the system. Those already on the rolls fear their benefits may be cut. Those nearing retirement fear they will be shortchanged. And younger people fear the system may not exist when their rightful turn comes to collect.

President Reagan therefore was prudent to create a 15-member blue-ribbon commission, including five nominated jointly by Senate Majority Leader Howard Baker and Minority Leader Robert Byrd, and five by House Speaker Tip O'Neill and Minority Leader Robert Michel. A bona fide expert--Robert J. Myers, chief actuary for 23 years and a former deputy commissioner of the system--took on the role of executive director.

Obviously, there is an effort to achieve a delicate political balancing-- and given the wide range of views on the seriousness of the problem, 15 differing opinions are a worst-case possibility. But Greenspan hopes to get a bipartisan consensus. "We will all have to swallow hard on some of the views we held previously," he told the first commission meeting in February.

Greenspan told me that commission member Sen. Daniel P. Moynihan (D-N.Y.) had suggested to his colleagues that each might be entitled to his own "value judgments," but not to his own set of facts.

But Myers observed that what makes the problem so frustrating is that many of the "facts" depend on economic assumptions: "The one thing that everybody agrees on is that we have a problem in this decade, because if the present law isn't changed, the July 1, 1983, checks for the Old Age and Survivors Insurance fund (the one that pays old-age benefits) cannot be met in a timely manner."

In other words, the Treasury will not have the money next year to mail out Social Security checks. (They would be delayed, until there was enough money in the till.) That is an established fact, the big problem facing not only the commission but Congress and the Reagan administration.

Myers suggests that this situation may be avoidable--here the word "may" is stressed--if Congress allows the old-age fund to continue borrowing from the Hospital Insurance fund (which helps finance Medicare).

But then, differences of opinion emerge, and the "facts" get cloudy. If one assumes that the economy will be lifted out of recession into a smart recovery, the old-age fund could probably squeak by until the beginning of the 1990s, with no help beyond interfund borrowing.

With luck, the 1990s may be a low payout period for old-age benefits--because the retirement population would be based on the low-birth years of the 1930s --allowing the OASI fund to repay what it borrowed from the other funds.

But others argue that this is no solution. Is it wise policy, they ask, to assume a solid economic recovery? Shouldn't the OASI also have authority to borrow from the Treasury? What happens if the hospital fund, tapped for help by OASI, suddenly finds itself pinched? (Some think the hospital fund is headed for bigger trouble than the OASI fund.) What happens if inflation in the 1990s again increases faster than wages, instead of the reverse? (The whole Social Security system is based on the assumption that wages will rise 1.5 percentage points faster than the consumer price index.)

These are the questions the commission has to address on the immediate problems--without ever getting to the question whether or not there is a problem of solvency in the system in the years beyond 2020 and, if so, whether to deal with it now.

But the short-term problem is practically at hand. Congress will be under severe pressure to do something more than merely authorize indefinite interfund borrowing. The possibilities range from cutting the growth of old- age benefits to increasing the tax rate to cutting back on the generosity of the inflation adjustment system.

Myers seeks to soften public concern. "Too often," he says, "people think of Social Security like a bank: if its assets are less than its liabilities, it's finished. But in Social Security, there is no legal contract: the plan can be changed. The administration and Congress can make relatively small adjustments that will solve the problem, and I'm certain they will do it."