SAN FRANCISCO, SEPT. 2 -- California's stunning population gains, in combination with a now firmly established pattern of strong Republican presidential voting in the South, will turn the West Coast into the key battleground in future contests for the White House, according to a number of political scientists meeting here.

"The Democrats will have to carry California to win presidential elections," said David Mayhew of Yale University.

Interviewed at the annual meeting of the American Political Science Association that ended here today, a number of political scientists noted that the preliminary population figures released by the Census Bureau confirm a continuing shift in political power away from the Northeast and Midwest to the South and West that will influence the nation's political agenda into the next century.

For example, they said, the tilt in the balance of power toward California will intensify the partisan struggle for the loyalty of Hispanics, who historically have had low voter turnouts but whose population growth now clearly gives them the potential to determine the outcome of elections in California and Texas.

Richard Brody of Stanford University contended that "if Democrats can get Latinos {in California} to mobilize, it could break the Republican hold on the electoral college."

But David Brady of Stanford argued that the short- to medium-range prospects in California are for an increasingly strong Republican vote. "There are two kinds of immigration. One is low-turnout Hispanic. But people moving to California who vote are conservatives. Look at the housing market, it's people who can pay $350,000 for a house in San Jose. They vote."

The population shifts away from the major Northeast and Midwest cities -- once the centers of power in the Democratic Party -- will force other significant changes in politics and policies.

"Demography is destiny, but it is a slow-moving process," said Walter Dean Burnham, a political scientist as the University of Texas in Austin. This slow process, he said, in pushing the coalition structure of the Democratic Party "away from anything that looks anything like the New Deal coalition."

The population changes "are not going to help the Tip O'Neill wing of the Democratic Party," said Martin Shefter of Cornell University, referring to traditional Democratic interest group politics in which major players are unions, city political machines, ethnic organizations and government employees. While such groups lose influence over the nation's political agenda, new priority is likely to be placed on issues of importance to the high-growth areas of the Sun Belt, including new highways, water projects, coastal oil drilling and immigration.

The population shifts are, in addition, likely to increase the political isolation of the Northeast and Midwest cities that hold large and growing numbers of the "underclass" -- very poor, disproportionately black, families experiencing high rates of unemployment, illegitimacy and crime.

William Julius Wilson, a sociologist at the University of Chicago, has found that over 75 percent of the growth in the size of the underclass has been in 10 cities, all with either stagnant or declining overall populations as the middle class moves out, and fully half the increase has been in New York and Chicago. While the nation as a whole gained population in the last decade, Chicago's population fell by 9.3 percent and New York's declined by 0.5 percent.

Lawrence Mead, of New York University, said the continuing "isolation of the poor black population in declining cities" may intensify conflict over such policies and welfare reform. The political leadership in older, poorer cities is likely to become "more entrenched" in its opposition to such welfare reform proposals as workfare, but to simultaneously face a loss of support from a more conservative and suburban national population, he said.