Candidates zero in on vote-rich states. With the election just four weeks away, Dukakis and Bush are pouring resources into a handful of big states whose electoral clout will prove decisive

Fewer than 10 big states, including California, Texas, and Illinois, now hold the key to the presidential race. Michael Dukakis, trailing George Bush in the stretch drive toward election day, according to polls, will focus on a handful of hotly-contested states with large numbers of electoral votes. Experts say those states will decide the election.

Vice-President Bush has pulled ahead in most Southern and Rocky Mountain states. Together, they give him a large, stable political base that puts him more than half-way to the White House.

Governor Dukakis has no comparable base. The latest survey by the Field Institute shows him with a solid lead only in Minnesota, West Virginia, and the District of Columbia. Even Massachusetts is somewhat shaky.

Without a strong base, Democrats are forced into a high-stakes strategy that relies on a near-sweep of every large state, from New York to California. Even the loss of two or three large states could put Mr. Dukakis at risk.

This grim outlook for Democrats comes at a time when the Dukakis forces are rejuvenated by their own internal polls, which show the race tightening nationwide. The campaign got a lift from last week's vice-presidential debate, where Democrat Lloyd Bentsen was widely perceived as the winner.

Yet experts doubt that Senator Bentsen's edge over Sen. Dan Quayle in the debate will be enough by itself to salvage Democratic hopes.

Claibourne Darden Jr., an Atlanta pollster, says Texas illustrates the magnitude of Dukakis's problem with only 28 days remaining in the campaign.

Mr. Darden notes that Dukakis picked Mr. Bentsen as his running-mate with a primary thought in mind: to break the Republican hold on the Lone Star State, and to bolster the ticket in other Southern states.

But Bentsen's impact appears to be falling short.

``Texas is now four to seven points for Bush, depending on the poll,'' Darden says. ``There's a feeling that rural folks are expressing to me in the South, that unless they [the Reagan-Bush White House] are not doing something right, there's no reason for someone else to have their job.''

Among the major states, Florida (21 electoral votes) is solidly in Bush's column. But the others, including Texas, are battlegrounds:

California (47 electoral votes). The biggest prize of all. A dead heat, with the trend slightly toward Bush. Absentee ballots could determine the winner.

New York (36 electoral votes). Dukakis's best big state.

Pennsylvania (25). Currently a toss-up. A Democratic must.

Illinois (24). Another tossup. This is Dukakis's foremost opportunity among the major Midwest states.

Ohio (23). Currently tilts slightly toward Bush in the polls.

Michigan (20). Another leaner toward Bush.

New Jersey (16). Bush's lead appears to be widening.

Lee M. Miringoff, director of the New York-based Marist Institute for Public Opinion, says Dukakis still holds a lead in the Empire State. But Democrats are at best only six points ahead, and one recent GOP poll showed Bush even.

Dr. Miringoff and other analysts say that the battle for the big states has a different complexion than in 1984, when Ronald Reagan led the GOP ticket.

Mr. Reagan's overwhelming victory in '84 came from across-the-board support, but in big industrial states he most notably won the backing of millions of ethnic Roman Catholics, the so-called Reagan Democrats. They liked Reagan's conservative social agenda and his get-tough defense policies.

Though he often espouses the same policies, Bush doesn't attract as many of these voters as Reagan did. Nor is he as popular with good-old-boy Southerners. His monied, prep-school background leaves many Reaganites cold.

But Bush has strengths of his own, especially among independent, middle-class voters concerned about the future of the economy, crime, and national defense.

Analysts say that Dukakis now gets support from about 83 percent of the Democratic vote, while Bush can count on over 90 percent of the GOP vote. The cross-over voting of the Reagan years is waning, and 1988 is shaping up as a classic Democratic vs. Republican struggle - with independents holding the balance.