PRESIDENT REAGAN was elected in large part because voters in 1980 wanted some discipline imposed on the federal budget. They felt, with reason, that the budget was out of control. The voters had a hard time, however, specifying where spending should be cut: in polls, they showed themselves quite happy to cut welfare and foreign aid, but favorably disposed to increasing spending for almost everything else. So when Mr. Reagan came to office, both personal inclination and political need set him a difficult task: to decrease domestic spending in a politically acceptable way.

The Congressional Budget Office's report on spending cuts, matching spending for 1982-85 as it would have been under 1981 laws and as it is scheduled to be under 1983 laws, helps gauge how well Mr. Reagan has performed--or persuaded Congress to perform--this task. Overall, spending has been reduced, for four budget years, a total of $110 billion. That's a lot of money--or seems like it until you reflect that deficits are projected as far as the eye can see on the order of $200 billion each year. Some $26 billion of the reductions came in Social Security and related programs and another $18 billion in health programs, mainly Medicare; most of these savings were in non-means-tested programs, which is to say those to which people are entitled regardless of income. Larger spending cuts came in categories most of whose programs are means-tested, which is to say that only people with low incomes or particular problems are eligible for benefits.

Budget Director David Stockman, in the famous Atlantic monthly article, expressed the hope that the administration would be as successful cutting programs backed by weak arguments as it was in cutting programs backed by weak constituencies. These figures suggest that that hope hasn't been realized. The non-means-tested programs, for which almost 100 percent of the electorate is eligible, suffered smaller cuts in dollar terms, and very much smaller cuts in percentage terms, than the average of the domestic programs in the CBO report. Cuts tended to be concentrated in means-tested programs, like unemployment, food stamps, child nutrition and welfare, which affect only a minority but sometimes are very important to them.

Yet Mr. Reagan and Mr. Stockman may have made weak constituencies strong. A significant part of the "gender gap" that dogs Mr. Reagan is the fact that a larger proportion of women than men have been affected by cuts in means-tested programs. Single women heading families--a low-income group-- have been particularly hard hit. Voter turnout among low-income Americans during the 1970s was low and falling; fewer and fewer politicians used strategies that depended on the low-income vote. By the late 1970s, only a bare majority of eligible Americans voted for president, and far fewer voted in off- years. Politicians seem to have concluded, understandably, from these facts that low-income people didn't care much about the programs that were intended to benefit them and that it made more sense to respond to the demands of the above-average-income electorate to prune government spending.

But now that spending has been cut, every sign indicates that voter turnout is up among the poor and, especially, the black. For the first time in 20 years, turnout for an off-year election was up rather than down in 1982, and the Democrats were the beneficiaries in most states. Moreover, the CBO report points out that reductions in cash benefits will be much greater, under current law, in 1984 and 1985 than they were in 1982 and 1983. If it's true that such reductions stimulated a lot of low-income people to vote in 1982, the political effect by November 1984 may be greater--and more damaging for the Republicans. Mr. Reagan may have done what the voters of 1980 wanted him to, but that may not please the voters of 1984.