LIKE ANYONE who stages a mass demonstration in Washington these days, the organizers of the Solidarity Day rally run a number of serious political risks. No matter how many people show up --the 100,000 that the labor unions organizers modestly estimate, the 200,000 that the D.C. and Park police now expect or even thousands more--someone will say that there really weren't that many, or if there were, there should have been more. Someone else will note that the hundreds of thousands of dollars spent by the organizers and participants to protest the Reagan administration's policies might have been spent instead on direct help to the losers from those policies.

So what gains can be expected? Mass demonstrations long ago became decidedly ho-hum events on the Washington scene. Even so upright and All- American a group as the nation's farmers discovered two years ago that it takes more than signs, songs and roadblocks to sway many votes in Congress. Today's rally will draw the attention of many in Congress who already sympathize with its goals, but there won't be anyone hanging out of the White House windows to watch.

If there is, however, one important audience for today's demonstration it is--oddly--the participants themselves. Solidarity Day is important to organized labor and to the loose coalition of civil rights, women's rights and social action groups that will join in presumably common protest. It is important as a first test--in the era of scarcity--of the strength of the ties that bound these groups together in happier days when free-flowing federal money made compromise easy.

As the members of each group march today, they will be looking around at the other marchers. Perhaps they will ask themselves if they have more in common than indignation over the administration's disregard for each of their particular pet causes. If the answer is yes, the next step may be exploring ways to compromise differences and make common concerns felt on the political scene. That would be an important outcome. One of the curiosities of the Reagan presidency is that while the things Mr. Reagan does raise a lot of individual emotions, so far there has been nothing like an organized opposition to his program. What will be significant in today's rally is not the number of participants, but the extent to which their coming together produces a coherent sense of purpose.