Every successful presidential candidate needs at least one secure geographical political base. President Reagan is fortunate to enjoy two, in the West and South, and he has done the most with them in his reelection campaign against Walter F. Mondale.

Reagan's lead looks commanding from a distance. Viewed regionally, it is even more impressive when probable results are translated into electoral votes.

Take a map of the United States. Draw a line from Canada to Mexico along the eastern borders of the Dakotas, Nebraska, Kansas, Oklahoma and Texas.

West of this line, in what Reagan pollster Richard B. Wirthlin calls "the greater West," are 18 states with 162 electoral votes, a computation including Alaska but not Hawaii. Reagan's narrowest margin in any of these 18 states is in Oregon, where he leads by 11 percentage points.

Draw another line across the northern borders of Arkansas, Kentucky and Virginia. South of it are 11 states with 118 electoral votes. Reagan's narrowest lead in these states is in Georgia, where he is ahead by 13 points.

Together, these states have 280 electoral votes, 10 more than the number needed for election, and Mondale is barely within hailing distance in any of them. In the megastates of these two regions, Wirthlin's trackings show Reagan with a lead of more than 30 points in Texas, nearly 30 in Florida and 13 in California.

This geographical exercise demonstrates the difficulty of Mondale's position but is a comment on more than his candidacy. What has happened in 1984 is a reassertion of a long-term conservative and Republican presidential trend in the Sun Belt that was disguised by the emergence of Jimmy Carter.

The West has been reliably Republican for three decades. If the Barry Goldwater debacle of 1964 is excluded, only three of the 18 states in Wirthlin's greater West -- Nevada and New Mexico in 1960 and Washington in 1968 -- have voted for a Democratic nominee in seven presidential elections.

The South, once a Democratic bastion, was heading in a similar Republican direction in presidential elections until the fall of Richard M. Nixon and the rise of Carter. While Carter has been blamed for everything from sunspots to the intelligence failings of the Reagan administration, it is hard to fault him as a regional candidate.

Carter won the presidential election on the strength of the South in 1976, and in 1980 gave Reagan a race in the region that no northern Democrat could have duplicated. He demonstrated that the Democrats need candidates competitive in the West or the South to prevent Republicans from unifying the Sun Belt and holding a lock on presidential elections.

The Reagan and Mondale campaigns differed on the geographic realities. Reagan pursued what Wirthlin calls "the classical strategy of first securing your base, than pursuing the margins." But Mondale made no attempt to nail down his natural Democratic base in the Northeast and industrial Midwest.

"We've never considered the geography of this election to be very important," Mondale campaign chairman James A. Johnson said last week. "Mondale has to run in all regions."

Consequently, Mondale lost time in the South that might have produced fruitful results in the Northeast. He also devoted heavy resources to Reagan's home state of California, where early polls fostered an illusion of Reagan vulnerability and where Democratic congressmen wanted a strong party effort to defeat a Republican reapportionment initiative.

The initiative may lose, but Reagan will carry California as he always has done. Californians hold such strong views about Reagan, for and against, that it is difficult to move them. The same effort in New York and Pennsylvania might have put Mondale ahead there instead of struggling to catch up.

This unusual strategy of trying to run everywhere has not given Mondale a chance to run strongly anywhere. As Edward J. Rollins, campaign director of Reagan-Bush '84, puts it: "The only reason we have a potential for a 50-state election is that they didn't solidify their base at the outset." Reaganism of the Week:

Speaking at the University of Portland last Tuesday, Reagan said: "America will never stop. It will never give up its mission, its special mission. Never. There are new worlds on the horizon, and we're not going to stop until we get them all together."