Two federal employe unions representing 1.5 million workers staged a rally yesterday in front of the White House to dramatize how fed up they are with their treatment by President Carter and the Congress.

The turnout, however, demonstrated as much about the nature and role of the federal work force, and the political bind facing its union leaders, as it did about their outrage over proposed changes in pay and benefits.

Park police estimated the crowd at from 1,000 to 3,000; union leaders claimed 3,000 to 5,000. An estimated 1,500 of those were bused in from Philadelphia, New York, New Jersey, Baltimore and other cities by union officials.

The weather was shimmeringly perfect, the park within easy walking or Metro-riding distance of tens, possibly hundreds of thousands of potential sign carriers for the cause, and the rally took place during the lunch hour.

Yet, as Veterans Administration worker observed dryly as he returned to his office across the street, "Save the Whales got bigger support than this."

And, as one spokesman for the largest federal union put it, in terms of the national political mood, "We'd be damn fools not to admit that our clout is not very great right now."

Still, this turnout, coupled with others of varying size in cities around the country, could be the start of a new political awakening by beleaguered government workers, they said hopefully. They noted that this is the first time all the rival federal unions have joined forces.

"You can't go by the numbers in the Washington area," said Jerry Klepner of the National Treasury Employees Union, which has 5,000 to 6,000 members, or about 10 percent of this area's total unionized work force.

"There's a distinction between the people in the field and here," he said, "because Washington houses greater numbers of high-ranking professional and technical employes here.

"Federal workers don't turn out. They never had to," said John Reeder, a commodities analyst for the International Trade Commission and one of the demonstrators. "But this is just the beginning. Carter doesn't know what he's in for."

The employes claim they have been made "scapegoats in the war on inflation" and "victims of budgetary polics." Congress, for example, is attempting to reduce the annual cost-of-living raises for federal retirees, and merge the civil service retirement system with the less-attractive Social Security system.

But they reserve their saltiest epithets for President Carter and his series of proposals to increase executive control of the bureaucracy and make his work force "more productive and efficient."

The latest, just sent to Congress, would reform the federal pay system in a way expected to reduce the pay raises of many federal workers for several years, to bring their pay more closely in line with their private sector counterparts.

In planning their strategy against the pay plan, however, union leaders face an unpleasant choice, according to both union and administration sources.

If they take a moderate approach, accepting some parts of the proposal and negotiating changes in others, they might anger their membership and become vulnerable as leaders. But if they maintain all-out opposition to it, they risk losing the whole system on which their current pay is based, that it, comparability with the private sector - a principle that the unions favor.

By the fall of 1980, just before the elections, the current system will prescribe a raise of 13.1 percent for federal workers - allegedly to bring them even with private industry, one administration official said. "Even if that was a fair comparison with industry, there is no way - politically - they would ever get that."

Union officials said yesterday they have not settled on either a legislative strategy or a candidate to endorse for 1980. CAPTION: Picture, Federal workers express their feelings in protest rally in front of White House. By Frank Johnston-The Washington Post