Three weeks ago, I wrote a piece entitled "Twelve Long Blocks," about a woman on welfare who had such difficulty feeding her four children that she often went without food herself.

Although I did not expect it to be an overwhelmingly popular column with readers, I felt it was my duty to write it to be faithful to some of the places I go and the things I see in Washington.

One of the factors of our nation's greatness is that it is so dynamic. Unlike older countries that are mired in tradition, ours can change direction rapidly. Twenty years ago, after some startling revelations about Americans going hungry, our government with the full support of our citizenry declared war not only on hunger but also on poverty itself. Now, in the mid-'80s, our attitudes are very different, and it seems we've lost not only compassion, but even tolerance for the poor.

Some of the letters I received in response to the article make that point very clear. The search for new ideas is healthy. So is the debate it produces. But certainly in a democracy, people ought to be able to disagree without being disagreeable. But some of the letters I received after writing "Twelve Long Blocks" crossed all lines of rational and democratic responses, and some could only have been inspired by meanness.

"What do you black niggers expect," one letter began, "welfare from cradle to grave?" In another letter, a correspondent chose to speak for the larger segment of the country while exhibiting a limited knowledge of eugenics: "White Americans are fed up with the inferior black."

One of the interesting things about these two responses is that both of these correspondents assumed that the woman I wrote about is black. It apparently never entered their minds that she could be white. Although she is indeed black, I could have just as easily written an article about a white woman with three children living in Minneapolis. But that picture doesn't correspond to the popular image most people have of welfare recipients. But it is a false image; indeed, a majority of welfare recipients are white, and it is also in their behalf that I wrote "Twelve Long Blocks."

The two responses above were not the only ones of that nature.

Another letter took the black middle class to task: "Why don't you blacks with ample income start a program to educate these 'poor blacks' to not having so many illegitimate children? . . . Stop writing about the poor blacks on welfare and get out and do something to help them into a mood to correct their status."

This correspondent fails to realize that blacks through churches, community organizations and individual efforts are doing things to help, and have done so down through the years. But many of these efforts are unfolding behind the scenes.

Labeling me a left-winger and at the same time tossing in a peculiar barnyard parallel, another correspondent's letter said: "There is an old saying that you can take a pig out of the mud, dress him up, give him a bath, and spray him with perfume. But once he is let out, he will go right back to his old familiar surroundings, wallowing in the mud. The same can be said for the kind of people you and your left-wingers continue to support."

Judging from some of the letters I received, one thing is clear: people resent having to work hard while others, they feel, can sit home doing nothing and receive checks. While I can understand their feelings, I also know that there are very few welfare recipients who would not prefer to make a decent living by their own hands. It is difficult for anyone who has not been poor to know the demoralizing, even dehumanizing effect it has on the human spirit, especially in a generally well-off society.

Recalling how recently we seemed to share a national benevolence and to take pride in our nation's domestic and international efforts to raise the quality of life for the poor, I find it particularly unsettling to see how quickly some are able to slide into society's oldest excuse for uncharitableness: blaming the victim.

My mail tells me the era of national generosity is past. And with its passing it is not only our willingness to aid the needy that has been diminished; we have, too.