Horace Busby, the wise old former Lyndon B. Johnson aide who now explains Washington and national politics to business clients, is the inventor of the concept of the "Republican lock" on the presidency.

Busby pointed out a couple of years ago that in the Sunbelt states, which now control almost half the Electoral College votes, Democratic presidential victories have become such rarities that under almost any circumstances, the Republican nominee has to be considered the favorite for the White House.

Busby himself stretched the theory beyond the borders of the Sunbelt to include such Republican-inclined states as Maine, Indiana, New Jersey, Ohio and the Dakotas -- all of which saw notable Democratic victories this year. But his main point comes into focus if you concentrate on the 26 states of the South and West, which have 264 electoral votes -- just six less than a majority -- under the new apportionment.

In the eight presidential elections from 1952 through 1980, one of those states, Arizona, never voted Democratic; 9 voted Democratic once; seven, twice; one, three times, and four, four times. That means that of the 26 states, 22 have gone Democratic four times or less in the last eight presidential elections. That tilt is what Busby calls the "Republican lock."

It's a good theory, but it hardly fits the political mood here in Busby's home state, or in much of the rest of the South, the Southwest and the West. Throughout the Sunbelt, Democrats are celebrating their midterm election victories and are looking forward to 1984.

This was the best election Texas Democrats have had in the two decades since Sen. John G. Tower (R-Tex.) shattered their monopoly on major office. A massive turnout of farmers, workers, Anglos, blacks and Chicanos swept Gov. William Clements (R) out of office, elected State Attorney General Mark White (D) to replace him, put a slate of mostly liberal Democrats into every other state office, and gave Democrats the three new House seats Texas gained in reapportionment.

Texas is the brightest star for the Democrats in the Sunbelt, but it is not their only one. Since Ronald Reagan was elected in 1980, they have lost one Senate seat in the region, but have made a net gain of four governorships and 19 House seats.

In January, Democrats will control 21 of the 26 governorships in the South and the West, 23 of the 52 Senate seats, and at least 135 of the 212 House seats. (Two more House seats, both now held by Democrats, will be filled in special elections in Georgia later this month.)

That hardly looks like a "Republican lock," but as Busby pointed out, the South and the West are very comfortable voting one way for governor and Congress and a different way for president.

So the real question is whether the Democrats can convert their grass-roots strength in this region into a presidential victory in 1984. That depends in large part on who is running. Jimmy Carter captured the southern half of the Sunbelt for the Democrats in 1976, but lost most of it, along with the West, in 1980.

Two southerners, former Florida governor Reubin Askew and Sen. Ernest Hollings of South Carolina, have been testing the presidential waters. Sen. Gary Hart of Colorado and Sen. Alan Cranston of California are ready to jump into the race as regional candidates from the West, and Rep. Morris K. Udall of Arizona is also making noises about running again.

Given Busby's logic, there is a powerful argument to be made for the Democrats' nominating someone who has a special appeal to the Sunbelt. But that is not necessarily a southerner or a westerner. Sen. John Glenn of Ohio is the kind of moderate, pro-defense Democrat that the South and the West like to elect. He has worked the region hard enough this past year so that he can accurately be said to be pursuing a Sunbelt strategy.

Former Vice President Walter F. Mondale has kept alive his Carter connections, for what they may bring him in some southern states. He has strong allies here in Texas, including people very close to the incoming governor.

Sen. Edward M. Kennedy of Massachusetts is regarded by many Democrats in the South and the mountain West as a candidate who cannot carry their states. But he has strong support in California, and his hold on blacks and Chicanos is such that he cannot be dismissed in any state where they form a growing voting bloc.

The 1984 contest will begin in the Midwest (with the Iowa caucuses) and the Northeast (with the New Hampshire primary). But the inexorable power of numbers means the Democrats cannot ignore the South and the West. On the basis of their success in 1982, there are many Democrats here who think their party should forget about Busby's "lock" and find the candidate who has the key to doing what so many other Democrats have proven can be done: win in the Sunbelt.