Although George Bush has been on the offensive against Michael Dukakis since winning the Republican presidential nomination, the heart of his campaign strategy is defensive: He has to protect the Republican base built by Ronald Reagan.

If Bush secures the GOP's natural base in the South and West and wins his share of states in the Midwest and Northeast, he'll be elected president, according to campaign officials and other analysts.For the most part, Bush is following Reagan's game plans from 1980 and 1984, when he pried away independents, white Southerners and Northern white ethnic, Roman Catholic and blue-collar voters from the Democrats to win in landslides.

No one in the Bush camp says he'll have it as easy against Dukakis as Reagan did against Walter Mondale, but even Democrats acknowledge that the Republicans start with an advantage in the Electoral College.

"Coming into this election, the Republicans are in the strongest position they have been in this century," said Democratic analyst Horace Busby, whose predictions of the Electoral College count in 1980 and 1984 were the most accurate among his colleagues.

According to Busby's calculations, Franklin D. Roosevelt won 88 percent of the electoral votes in his four elections, a base that aided Harry S. Truman in 1948.

By contrast, in winning four of the last five presidential elections, the Republicans have won 86 percent of the electoral vote, Busby said, "which shows what the Republican candidate and party has as a base this time."

Busby, a former aide to Lyndon B. Johnson, advances the theory that the Republicans have a "lock" on the Electoral College, which makes it almost prohibitive for a Democrat to win.

A candidate must win 270 of the 538 electoral votes to be president. Each state has the same number of electoral votes as it does senators and representatives. Whoever carries a state wins all its electoral votes.

What gives rise to Republicans' optimism about 1988 is the fact that 23 states have voted Republican in the last five elections, and those states have 202 electoral votes. Thirteen states, or 152 electoral votes, have voted Republican all but once. The nation's 12 most populous states, which have 279 electoral votes, were carried by Reagan in 1980 and 1984.

"There is no inherent reason for this election to be close," Busby asserted.

Nevertheless, Bush is nowhere near the same level in the polls that Reagan was this time four years ago. Reagan led in every region and was considered so invincible in the South and West that he began to campaign in traditionally Democratic strongholds to see if he could win every state. He lost only Minnesota, Mondale's home.

Because many states are too close to call and a lot of voters are undecided, Bush is spending time all over the country, particularly in the most populous states that are viewed as the battlegrounds.

Central to Bush's strategy is winning at least two of the three Sun Belt mega-states - California, Texas and Florida - which combine for more than a third of the 270 votes needed to win. Strategists on both sides think Bush is ahead in Florida. Texas is closer, because of Dukakis' selection of Texas Sen. Lloyd Bentsen as his running mate, and traditionally Republican California is a toss-up.

After these states, Bush is trying to nail down the South, where almost all the states currently lean Republican.

The other natural base for the GOP is the Rocky Mountain states, the most Republican region in presidential politics. Added to that base are traditional GOP farm states of Kansas, Nebraska and the Dakotas.

Missouri is considered a toss-up and already has received a lot of attention from Bush, who beat Sen. Bob Dole of Kansas on Super Tuesday.

Bush also will go after some states in the Midwest and Northeast where many Democrats live. Illinois, Michigan, Ohio and Pennsylvania will be battlegrounds. New York leans to Dukakis, while New Jersey leans to Bush. Some New England states are reliably Republican.

Reagan's pollster, Richard B. Wirthlin, said he expected the president to spend the most time helping Bush in the South and in California.