PRESIDENT AND MRS. REAGAN have agreed to serve as honorary chairs of the United Way in that organization's centennial year, 1987. Founded by four clergymen in Denver, the organization raised $21,700 in its first campaign. Last year, a record-breaking $2.33 billion was contributed; this was a 9 percent increase over 1984. United Way hopes to double this figure over the next few years, and the president has challenged them to do so even faster.

The extent of private charitable giving in this country is something to be proud of. Whether responding to natural disasters, such as the Mexican earthquake and the West Virginia floods, or helping in an international crisis such as the African famine, or simply helping neighbors in need, Americans give generously. Even when times are hard in this country, the United Way has reported, gifts continue to grow. In the depths of the recent recession, for example, the United Way reported an increase in individual gifts, even in such places as Detroit and Flint where unemployment figures were staggering.

The president made an important point in accepting the United Way chairmanship. In addressing the needs of the poor, he said, "the government alone is not the answer." It is certainly true that private charities and the work of volunteers are urgently needed. But it is also true that the government must continue to provide for the bulk of those needs and that the private sector alone is not the answer either.

Think back to the beginning of this century, before the social welfare programs run by federal, state and local governments were instituted. Private charities cared then for the sick and the poor, the elderly, the mentally ill, the handicapped and the homeless. Now, as citizens and taxpayers, we do much of this through public agencies. Private groups complement and enhance these programs, but the government is the main provider.

This country is moving more deeply into a period of severe budget retrenchment. As a review proceeds of the public expenditures on behalf of the poor, it is important not to be captivated by the hope that private charities will pick up the slack. The United Way -- or any other charity -- with all the hard work and good will in the world cannot be expected to underwrite such comprehensive and expensive programs as AFDC, food stamps and Medicaid. To the nation's poor, who benefit from the generosity of private charities, the public programs are a matter of survival. Legislators grappling with budget decisions cannot avoid that central fact.