It really hit me when I was talking to my dentist about President Reagan's cuts in social spending. He's 41, an intelligent fellow, but he had never even heard of a county poorhouse. For him and for the bulk of other Americans, the social safety net seems always to have been in place.

That bit of conversation made me realize that today, exactly half a century after Franklin D. Roosevelt took the first of his four presidential oaths, I am among a rapidly dwindling minority --for just about all of us are on the far side of 70 --who remember pre-FDR America and who have vivid memories of that first New Deal year that turned it all around forevermore.

Rome wasn't built in a day and neither was the New Deal. But certainly March 4, 1933, was one big bang historically, beginning with Roosevelt's "we have nothing to fear but fear itself." Those words, wrote the Secret Service chief, were "like an injection of adrenalin in the veins of public morale. The confidence, the courage, the boundless vitality of the new president, reached out and infected the whole country with hope. So far as the spirit of the thing was concerned, the depression ended right there."

Of course, it didn't. The multi- billions the government eventually poured out in a vast attempt to spend the nation out of the Great Depression did change the national mood and did produce millions of jobs, but the economic malaise did not totally disappear until America began to arm the allies and to prepare for its own involvement in World War II.

In retrospect, the Roosevelt era can be neatly divided into the pre-war years of recovery and reform and the war period. But the recovery-reform years were a maze of good and bad ideas, experimentation, political expediency, scandal, fumbling, first-rate and third-rate officials, divisions and bickering, presidential leadership and presidential demagoguery. At the time, it seemed very much slapdash, not the measured cadence of history books.

Roosevelt came to office with no coherent policy. He had called for "a New Deal for the American People," a hybrid phrase from Teddy Roosevelt's "Square Deal" and Woodrow Wilson's "New Freedom." He started out for a balanced budget and his first big congressional victory was over the American Legion on cutting veterans' benefits, along with government pay. All that was soon reversed, and modern-day big government came into being. FDR was not doctrinaire; he improvised, learned from mistakes, and he was pretty frank about it, too. He hated to fire people, so he resorted to bureaucratic layering; he simply created a new "temporary" agency with his own man in charge in order to get around some permanent piece of government he couldn't bend to his will.

But the central fact was that the United States had gone through an economic wringer in the nearly 3 1/2 years between the October 1929 stock market crash and that March 4. With a quarter of the work force jobless, state, local and private agencies simply couldn't come even near coping with demands for help. The county poorhouse, for generations a fixture all over the nation as a refuge for the unfortunate among us, was overwhelmed, as were the Salvation Army, church missions and private charities in the cities. Since Herbert Hoover resisted getting the feds involved in direct "relief," the voters threw him out of office. So they got FDR.

All the banks in the nation were closed by Inauguration Day, lots of them simply bankrupt, with countless savings accounts gone down the drain. That Inauguration Day, some Washington hotels warned guests they wouldn't accept out-of- town checks "due to the unsettled banking conditions throughout the country."

The New Deal's famous "First 100 Days" ended on June 16 when an exhausted Congress, which FDR had called into special session beginning March 9, trooped home after passing 15 major laws, just about all of them exactly as he had proposed them--to rescue banks, stop home and farm mortgage foreclosures, abandon the gold standard, put a floor under agriculture prices, create a national relief system, a public works program and a civilian conservation corps for the jobless, bring a measure of truth and honesty to the securities and banking world, including insuring bank deposits, and establish a sort of self-rule system for business and industry that included an end to child labor, a minimum wage and the right of collective bargaining.

Most of these ideas had been kicking around for years; insuring bank accounts, for example, had been a major item in William Jennings Bryan's 1896 campaign. But the moment and the man did not coalesce until 1933.

Some parts of these New Deal laws were struck down by a conservative Supreme Court; others simply were failures. But the sum of the 100 Days, as Walter Lippmann so well put it, was that the nation ceased being "a congeries of disorderly panic-stricken mobs and factions" and once again became "an organized nation confident of our own power to provide for our own security and to control our own destiny."

Roosevelt had this town jumping, from March 4 on. Lights literally blazed late in many a government office. That was when the capital of the nation truly and permanently moved from Wall Street to Washington, that's when Washington ceased to be a sleepy southern town.

It was the young New Dealers who rediscovered Georgetown, buying up and remodeling its slums. People who think the big change came with Jack Kennedy simply are wrong. One thing, however, that really didn't begin to change until the war was racial segregation in Washington; the city was under the thumbs of southern potentates in Congress, and FDR needed their help. Yet the rising economic tide sufficiently lifted all boats that grateful blacks made the historic voting shift from emancipation Republicanism to New Deal Democracy.

By the time I got here in September, the country was up on its feet again, and frightened Republicans had recovered their voices of alarm. FDR used the radio, though quite sparingly by current standards, to go over the heads of the press barons who so long had molded public opinion and made and broken public officials. Just about every journalist (including very few women) who wanted to could squeeze into the Oval Office for Roosevelt's regular Tuesday morning and Friday afternoon press conferences. For youngsters like me, standing way in the back while FDR remained seated behind his desk, it was hard to hear his words. But I knew I was in on history-in-the-making.

The atmosphere truly was infectious. It's a fact that the bulk of the working press quickly became New Deal liberals in outlook, and the inheritance of that remains to this day. I think it sprang simply from admiration of the New Deal's initial success and of FDR personally in his relations with the press. He truly was the "great comunicator" of this century.

The poorhouse was replaced by the welfare state, to put it simply, as the New Deal wove our first national social safety net. FDR's successors variously strengthened it, expanded it, sometimes too lavishly. Some presidents were able to alter parts of it or even to trim it back. But as recently as last November, Americans, most of them with no memory of the pre-New Deal era, declared emphatically with their votes that Reagan had gone far enough, maybe too far, and that they wanted no return to the era of the poorhouse.

By the end of 1933, Roosevelt could proclaim that "we are on the way" and it truly was believable. And on his 52nd birthday, Jan. 30, admirers all over the land flocked to some 6,000 birthday balls held as benefits for the Warm Springs (Ga.) Foundation, which FDR, a victim of polio, had made a symbol of hope for the physically handicapped.

It may not be in the same category as having been with Washington at Valley Forge or having heard Lincoln deliver his Gettysburg address. But just being in Washington, catching the fever and the flavor, for even half of that first New Deal year was to have seen modern-day America being created. You might even say I'm sort of proud of it.