The only vote that counts. In the Electoral College, Michael Dukakis appears to have a slim lead over George Bush. But many electoral votes remain undecided.

Michael Dukakis has moved ahead of George Bush for the first time in the all-important contest for electoral votes. According to a state-by-state survey, Governor Dukakis has gained an edge over Vice-President Bush in 17 states, primarily in the Northeast and along the Pacific coast.

Those states - which include California, New York, and Ohio - would give Mr. Dukakis 227 electoral votes to 214 for Mr. Bush. Election requires at least 270 electoral votes.

The Field Institute, which conducted the survey, estimates that Bush has an advantage in 27 states, many of them smaller states in the Rocky Mountain region. The vice-president also holds a commanding lead in the South.

Several large states, including Pennsylvania, Michigan, and Illinois, are currently too close to call, pollster Mervin Field reports. They could emerge as the swing states that determine the outcome of the election.

Mr. Field says that, based on his current state-by-state analysis, the coming election could be one of the closest in modern times.

Earlier this year, Bush held a wide electoral-vote lead over Dukakis. But during the past two months, his lead rapidly eroded as Dukakis locked up the Democratic nomination.

Field says, however, that much of the support for Dukakis appears to be anti-Bush rather than pro-Dukakis.

In California, where Dukakis currently sports a double-digit lead, Field explains:

``There is a two-word reason for Dukakis's strength in the polls: George Bush. Californians are voting `no' on Bush. But there has really been no comparative evaluation of Bush and Dukakis yet. So this is only Stage 1, and it is negative. But the contest has not yet been joined by any means.''

Democratic pollster Peter Hart says that despite recent gains by Dukakis, the Democrats need a breakthrough in the South this fall.

If Bush makes a clean sweep of the South and Rocky Mountain states, Mr. Hart says, it will put ``tremendous pressure'' on Dukakis in the rest of the country.

Hart told a breakfast meeting of reporters that seven major states are at the heart of Democratic hopes this fall. They are California, New York, New Jersey, Illinois, Michigan, Pennsylvania, and Ohio.

Democrats need at least six of those seven, Hart indicates. But if Bush sweeps the South, Dukakis may need all seven of those major states to win, Hart says.

For that reason, Hart insists that Dukakis needs a strategy to pick off at least three Southern states, such as Tennessee, Arkansas, and Kentucky.

Otherwise, Republicans will be able to target one or two Northern industrial states, such as New Jersey or Ohio, and pour in the resources to overwhelm Dukakis, Hart says.

``That would put Democrats in the position where they have to do extremely well in all of the tossup states to have a chance,'' Hart says.

Hart's analysis reflects the cold facts of the Electoral College, where each state is awarded to a candidate on a winner-take-all basis.

Democrats know that in the past five elections, 23 states with 202 electoral votes have favored the GOP every time. Another 13 have supported the GOP four out of five times. With that built-in advantage, the Republicans begin every presidential election with a strong edge.

Horace Busby, who once worked for Lyndon Johnson and now publishes the Busby Papers, calls this the GOP's ``electoral lock.'' Except for Jimmy Carter, who broke the Republican lock on the South, no Democrat has been able to put together an Electoral College majority since 1964.

The Field survey shows how the Democrats might finally succeed with a Northern candidate, and without most of the South. Dukakis's strength has its base in the Northeast. He runs well in the industrial Midwest, and in a band of states from Minnesota and Wisconsin down to Missouri. In addition, he has picked up a big lead along the Pacific coast.

Even without the South, Dukakis could pile up an electoral majority by adding Illinois and Pennsylvania to his base. But there is little room for error.

The need to fortify his position in the Midwest makes an Ohioan like Sen. John Glenn look attractive as a running mate for Dukakis. He helps Dukakis in the region, while also appealing to conservative Southerners.

Meanwhile, there was one bright spot for Bush in the Field survey. His strength in Texas solidified during the past month, shoring up his Southern flank.

But there are also worrisome signs. Field found Bush somewhat weaker in other parts of the South and in the Midwest.

State polls indicate that Bush doesn't get the same kind of broad support that Ronald Reagan enjoys among Southern whites, or among Roman Catholic ethnic voters in the industrial heartland.