If all politics really were local, as the saying goes, we would be congratulating the Democrats today. They came out of the 1984 election securely in control of most city and county governments, state legislatures, two- thirds of the governorships, the national House of Representatives (for the 33rd and 34th consecutive years), and even managed to increase their strength in the Senate in the face of President Reagan's landslide.

But ignoring the presidency in an essay on politics is rather like ignoring Leonardo Da Vinci in an art history course. While there is much to be said about the challenge facing the Democrats, let us focus for this moment on the Republican Party, which has established a preemptive claim now to being the party of presidential government.

There is no precedent in American history for a party so skimpy in its grass-roots strength exercising so near a monopoly on the highest office. It is a recent phenomenon. There have been 10 presidential elections since Franklin D. Roosevelt ended his unique four- term tenure. In the first five, between 1948 and 1964, the Democrats won three times and, by my calculations, received about 2 million more votes than the GOP. Of the five elections between 1968 and Tuesday, by contrast, Republicans won four and amassed a cumulative edge of about 44 million votes.

They achieved that historic breakthrough even though there have only rarely been times in that 16-year period when as many as one-third of the voters thought of themselves as Republicans -- and often much less.

How does one explain this turnaround? It is not a classic political realignr the two Reagan landslides -- very little to suggest and much to refute the notion that millions have accepted the president's invitation to join him in shifting their party identification.

The young voters, who gave Reagan his highest majorities, describe themselves most often to reporters and pollsters as anti-party people, deliberately splitting their tickets in an expression of personal autonomy. They could become the basis of conservative analyst Kevin Phillips' long-advertised "emerging Republican majority," but only if the performance of the next four years confirms their hopes of economic gain and if the Republicans nominate a candidate as young in spirit as Reagan was in 1980 and 1984.

Until that happens, Republican presidential victories will continue to rest on the ability of particular candidates to capture the support of "weak Democrats," those who retain an inherited or cultural predisposition to think of themselves as Democrats -- but who cast it aside for their presidential vote when convenience suits.

We know a lot from polling and interviewing about who these folks are. They are middle-class whites, often the middle-aged men and women who grew up in blue-collar homes where the memories of the Depression were fresh. They have worked their way free of the shackles of poverty and are conservative in that fundamental sense of wanting to preserve -- and if possible, expand -- the gains they have made.

They can be found most frequently in the inner-ring suburbs of the northern cities and in the South. They see the Democrats as a party of have-nots. It is no coincidence that they began to break away from voting Democratic -- on the presidential line -- in 1968. That was the first election in the modern era when inflation became a major issue. Inflation is a killer issue for those middle-class families, for it threatens their values -- their belief in hard work and saving -- as much as it erodes the value of their dollars. Since they cannot cope with the scourge of inflation themselves, they look to the president to solve it.

Richard Nixon was reelected in a landslide in 1972, when there was no sign of a Republican tide at all, after resorting to the extreme measure of a wage-price freeze to halt the resurgence of inflation. Jimmy Carter was driven from office in 1980 as much because he failed to deal with a renewed and virulent inflation as because he failed to free the Iranian hostages.

Reagan's victory Tuesday -- again without much evidence of a fundamental shift in the Republican direction -- had to do with the confidence the white middle-class gained not from the conquest of Grenada, surely, but from the seeming victory over inflation.

There is a clear implication from all this for the policy debate now beginning within the administration and among congressional Republicans. It is between the supply-side advocates of further tax cuts and full-throttle growth, on one side, and, on the other, those who put their priority on cutting the deficit by trimming entitlements and defense spending and, if necessary, raising taxes.

If the president wants to leave the next GOP presidential candidate a healthy political legacy, he will choose the course that promises to keep inflation down -- and make everything else secondary. If he keeps his eye on that target, he could live to see a Republican realignment -- even if it doesn't quite happen on his watch.