WHEN AMERICA HURTS, it comes to Washington to walk off the pain. Veterans marched because they were forgotten or, worse yet, hungry. The Klan marched because it was scared, and blacks marched because they were oppressed. People marched against the war on poverty and for African liberation and yesterday 250,000 marched some more. The old pain is back.

But it is diffused. It is not the moral pain of an awful war or the economic pain of mass unemployment or the daily pain of being black in a Jim Crow society. It is, instead, the pain proclaimed on the placards carrried by the Solidarity Day marchers. Some wanted jobs and some wanted import restrictions and some wanted lower interest rates and some, like the painters who breathe exotic fumes, want the administration to keep its hands off agencies such as OSHA.

A lot of the marchers came from unions that represent government workers. They want more jobs and they want raises. Ronald Reagan wants them to have neither. Some marched for reasons having to do with traditional union issues. An electrician from Pennsylvania said he was there to show his opposition to right-to-work laws. He and a buddy got lost, however, and decided to spend the afternoon drinking beer.

In a sense, this was a conservative crowd. This was not a group that had come to Washington seeking change. What they wanted was what they already had. What they feared was what was coming. They had no plan for change, for doing things differently. They wanted the status quo and the man many of them had voted for, Ronald Reagan, had crossed them up. He was going to give them the status quo ante.

For union officials down at the podium, this was a glorious day. If you asked what it all meant, they said that at the very least it meant that the union movement was vibrant, able to organize, far from dead. It could -- it just plainly could -- get 250,000 people down on the Mall in Washington, and that is no mean feat.

But the union movement, after all, is supposed to be able to organize mass demonstrations. Getting people already organized into unions -- into nationals and locals and chapters and God-knows-what-else -- into buses and down to Washington is not the same as organizing the poor for a march on Washington, or getting a couple of hundred thousand kids off a couple of hundred campuses to protest a war. The day was a success for American unionism mostly because it was not a failure. Unionism can afford few more failures.

For the marchers, the demonstration was a beginning. They could liken it to the beginning of the antiwar movement or the civil rights movement. They could say that this was the day it became respectable to criticize Ronald Reagan. They could say, as AFL-CIO president Lane Kirkland in fact did, that they had told us so, that Ronald Reagan's program was a prescription for pain. There is nothing as reassuring as looking backwards.

But the Reagan people, too, could look back. They could look back to history and see how all regimes need an enemy. Up to now, Reagan has not had one, but the unions are perfect -- maybe as good an enemy as the students at Berkeley who made Reagan a hero to many Californians.

Here, yesterday, were unions representing government workers. Here, yesterday, were unions supporting the air traffic controllers. Here, yesterday, were unions seeking protective legislation that means higher prices. Here, yesterday, were the unions who many Americans see as featherbedders and make-workers and obstructionists. As enemies, these will do fine.

So the battle is joined, and maybe for a while Ronald Reagan will win. He is the master communicator, while the unions speak lousy English. He cuts a swell figure, while the unions wear double-knits over big bellies. He sees the Big Picture, while the unions want only to protect their turf. He can argue, and undoubtedly will, that what's good for the unions is not necessarily good for America.

In the end, though, the outcome will hinge not on what Ronald Reagan does with the unions or they with him, but whether the pain that put the working man on the road to Washington spreads through society and becomes an epidemic. Ronald Reagan should have stayed in town and listened. Yesterday there were a whole lot of people saying "Ouch."