ON NO OTHER issue have the two major parties offered less enlightenment in this campaign than on Social Security. Every responsible politician knows that in the next six months the Social Security system will be changed in important -- and as yet undetermined -- ways. But few politicians are talking about Social Security responsibly. The Republicans have given us a folksy postman who tells us that the president has kept his promise to increase benefits. The Democrats denounce their opponents for opposing the program in 1935 and, they add, ever after.

Both parties are promising what they can't deliver. Responsible Republicans, and quite a few Democrats, favor some scaling back of benefit increases now scheduled for the future. Responsible Democrats, and quite a few Republicans, favor some increase in the increases already scheduled for Social Security taxes over the next few years. Both parties know that the Social Security trust fund will run dry unless such action is taken. Neither will say so out loud.

The parties know that a bipartisan commission, set up last year and chaired by economist Alan Greenspan, is scheduled to come up with recommendations on Nov. 11. The commission's members, responsible people of diverse views, are struggling to reach a bipartisan compromise; they are doing the work Congress and the president were unable to do in 1981 and 1982. For politicians of all stripes, such a compromise would provide much- needed cover.

Campaigners, especially in late October, tend to draw a narrow bead on Election Day. That is shortsighted politics. On Social Security, the two parties are taking risks by indicating broadly that they won't change the system at all when both know that changes are unavoidable, and soon. It is the last week of the campaign. President Reagan will be campaigning across the country, and various Democrats will be addressing the nation on television as well. It will be interesting to see if, in addressing Social Security, they keep in mind the long-term needs of the system as well as the short-term needs of their parties.