Democrat Michael Dukakis' chances of being elected president this fall depend on his ability to win some key states in the South and Far West, campaign officials and other analysts say.

The Massachusetts governor is expected to do well in several states in the Northeast and Midwest, analysts said. But these states will not be enough for the Democrats to win. In addition, Dukakis must capture a few states in the Republican base in the South and West."If Dukakis gets blanked in the South, the arithmetic of the Electoral College becomes very difficult," said Geoffrey Garin, a Democratic poll taker who worked for Walter Mondale in 1984.

GOP pollster Richard B. Wirthlin, who engineered Ronald Reagan's 1980 and 1984 wins, said, "It's very clear to me that the Democrats will have a difficult time winning unless they pick up at least 30 electoral votes in the South."

Winning the popular vote is one thing, but what really counts is the number of votes the candidate racks up in the Electoral College, a winner-take-all system based on the number of senators and representatives a state has. It takes 270 of 538 votes to win.

Republicans have had a lot of success in presidential elections, winning four of the last five and putting together a base in the South and West that makes it difficult for the Democrats to win. In effect, aside from a handful of small states, the Democrats have no natural base from which to start, which narrows their options for winning.

"We have a commitment to run a 50-state campaign," said Democratic strategist and former Dukakis operative Wendy Sherman, mouthing the campaign line that Dukakis intends to open offices in every state to compete with Bush.

But the stated goal of a 50-state strategy is necessary to keep the GOP off balance. The Dukakis campaign can scarcely afford to announce which states it will concentrate on, thus allowing Bush to focus on the key states Dukakis needs to win.

Dukakis' strategists said they'd narrow their targets in early October and would allocate the candidates' time, money and advertising accordingly. Some of that has taken place; the ads begun last week were only in populous states that are considered tossups by both camps.

The Democrats' best opportunities lie in the Northeast and Midwest, where many Democrats voted for Reagan in 1980 and 1984. To win, Dukakis must persuade them to return to their party.

In these regions, New York, Pennsylvania, Ohio, Michigan and Illinois are central to Dukakis' hopes. He also is counting on Minnesota, Wisconsin and Iowa.

But few of those states are reliably Democratic, and even if Dukakis won them all he'd still need some wins in the South and West to put together the necessary 270 electoral votes, analysts said.

That was part of the thinking behind Dukakis' selection of Texas Sen. Lloyd Bentsen as his running mate. The governor thought Bentsen could help win Texas' 29 electoral votes and ease concerns among Southern conservative Democrats that Dukakis is too liberal. Dukakis' choice of Bentsen also will force Bush to spend more time and money in his home state.

Dukakis also has other prospects in the South, such as Tennessee and Arkansas, but the pickings are slim, analysts said, unless he's able to convert more Southern voters between now and Nov. 8. Democrats still do well on the federal, state and local level in the South, and Jesse Jackson is expected to help turn out black voters who provided winning margins for many Southern Democratic Senate candidates in 1986.

In the West, Dukakis is mounting an all-out push in California, Oregon and Washington, three states with high concentrations of independent voters. Democratic strategists speak optimistically of his chances in all three states.

Pollster Garin said if Dukakis is shut out in the South, he'd have to win five of the following seven crucial states: California, New Jersey, Pennsylvania, New York, Ohio, Illinois and Michigan.

"That's a tall order," Garin said. "There will be a few other states, such as Missouri, where if you need extra electoral votes on the margin, states such as Missouri will provide them."

Because polls show that a majority of people who earn under $20,000 a year support Dukakis and those who make $50,000 and above back Bush, Dukakis strategists think the key voter group is middle-class Americans.