In the closing weeks of the 1988 election, voters can expect to see the two presidential candidates strolling California's shoreline and posing beside polluted waterways. Environmental issues are influencing the 1988 election more than they have any previous one.

Both Republican George Bush and Democrat Michael S. Dukakis are claiming they will do more to stop pollution than their opponent. Conflicting claims may neutralize the issue as a deciding factor in the election. But the winner is likely to be stuck with some ambitious environmental campaign promises to live up to.The environment has emerged as an important issue largely because the 1988 campaign lacks the issues, such as war or unemployment, that have always overshadowed it. Its importance also reflects the changing demographics of 1988: Both candidates are trying to win support from the large block of middle-class baby-boomers who have matured politically since Earth Day in 1970.

The two candidates sound alike on some of the environmental issues that have gotten the most media attention. Days after Dukakis called for canceling the Reagan administration's proposed sale of oil and gas leases off the coast of California in June, Bush called for a delay in the sale until after the election.

Both candidates say they would convene an international environmental conference to assert U.S. leadership on global concerns such as the warming of the atmosphere and the depletion of the Earth's protective ozone layer.

The differences in the stated positions of Bush and Dukakis have gotten less attention. Bush favors oil drilling in the Arctic National Wildlife Refuge, calving ground of the nation's biggest migratory caribou herd, while Dukakis opposes it. Dukakis wants to stop selling timber from national forests to logging companies for less than the costs of selling it. Bush does not.

On acid rain, both candidates contrast with President Reagan, who has insisted not enough is known about its causes and cures to justify tighter regulatory controls. Dukakis wants to cut annual emissions of sulfur dioxide, the main pollutant causing acid rain, by 12 million tons per year. Bush has called for reducing emissions by "millions of tons," without specifying how many.

Bush landed a body blow when he toured Boston Harbor, called it the nation's dirtiest waterway and blamed Dukakis for Massachusetts' long court battle for delays in sewage cleanup. Dukakis got less attention for his reply - that he had long since reversed the state's position, and that the Reagan-Bush administration had twice vetoed federal funds needed to clean up the harbor.

Dukakis has been less successful in drawing attention to Bush's role as chairman of Reagan's Task Force on Regulatory Relief, which canceled, delayed or weakened scores of environmental rules at the request of industry groups in the early 1980s. Dukakis calls the task force the administration's "wrecking crew."

While both candidates claim to be environmentalists, the established environmental groups are not completely satisfied with either man. They criticize Dukakis' performance on Boston Harbor, but they praise him for many other actions as governor, including his resistance to the Seabrook, N.H., nuclear plant.

Perhaps the bottom line on which candidate is the better environmentalist was written by the League of Conservation Voters, whose board includes the leaders of all the major environmental groups. The league gave Dukakis an overall grade of "B," while Bush took home a "D+."