Twenty years after President George H.W. Bush created the largest national marine sanctuary in the continental United States, banning offshore oil drilling along 276 miles of California coast from the Marin Headlands to Hearst Castle, the Obama administration is trying to fill in the missing piece.

The proposal would expand the Monterey Bay National Marine Sanctuary by just 1 percent. But the area isn’t just any small patch of ocean water. It’s the busiest, most high-profile section of Northern California’s coast, a highway for oil tankers and fishing boats, a recreational haven and a backdrop for millions of photographs — the waters directly west of the Golden Gate Bridge.

“People assume that the water around the Golden Gate is protected. It’s not,” said Maria Brown, superintendent of the adjacent Gulf of the Farallones National Marine Sanctuary. “That area is of unparalleled beauty. It is an iconic area.”

NOAA, the National Oceanic and Atmospheric Administration, will discuss the proposal at a public meeting Thursday evening at Fort Mason in San Francisco. If NOAA approves expanding the boundaries next year right up to the Golden Gate Bridge, the change could bring new rules affecting everything from fireworks shows near the bridge to a ban on personal watercraft such as Jet Skis off the Marin Headlands. It also could bring more protection for wildlife, higher international awareness for the sanctuary and more research dollars for everything from marine biology to explorations of 19th century shipwrecks submerged near the Golden Gate’s rocky cliffs.

The area that was left out of the sanctuary is 101 square miles of cold, choppy ocean — an area roughly twice the size of San Francisco — that extends north from Pedro Point in Pacifica, past Ocean Beach and the Cliff House in San Francisco to Point Bonita Lighthouse in Marin County. It is rich with seabirds, great white sharks and, in recent years, harbor porpoises.

But politics kept it unprotected.

When Bush designated the sanctuary in 1992, he approved the bite out of its boundaries after San Francisco city leaders raised concerns that their aging sewer system would be threatened with fines for releasing poorly treated sewage into sanctuary waters off Ocean Beach. In addition, Port of Oakland officials worried that they wouldn’t be allowed to dispose of dredged mud anymore near the bridge.

The two powerful players urged then-Gov. Pete Wilson for changes, and Wilson’s staff persuaded the Bush administration to leave the area off the sanctuary maps.

“I remember Wilson’s guy told us, ‘The only way this is going to work politically is if we have these exclusions.’ He was probably right,” said Dan Haifley, former executive director of Save Our Shores, a Santa Cruz environmental group that helped lead efforts to establish the sanctuary.

In July 1992, as the final plan was nearing completion, a little-known group called Coastal Advocates purchased a prominent ad in The New York Times calling the plan a sellout to polluters that needed to be rewritten. The group’s supporters included two environmental giants, former Sierra Club executive director David Brower and novelist Wallace Stegner.

Other environmental organizations panicked, believing that the sanctuary deal was in danger of collapsing. They thought that the only reason Bush, a Texas oil man, was supporting a ban on oil drilling over such a huge section of the California coast was that he was trying to win California in his November re-election effort against Bill Clinton and Ross Perot. The environmentalists reasoned that if the sanctuary wasn’t approved by the election, much of their leverage would disappear, leading to years of additional battles over offshore oil drilling.

“We said if we lose this now, we’ll never see it again,” Haifley said.

The Sierra Club, Surfrider Foundation, Defenders of Wildlife and other prominent groups supported the exclusion, and with support from then-Congressman Leon Panetta, Wilson, Tom Campbell, Sam Farr and other political leaders, the map gained momentum and won Bush’s signature two months later.

“At the time, this exclusion was considered temporary,” Haifley said. “I’m very pleased that this is finally happening.”

NOAA is proposing the change because the world changed, said Brown, who as Gulf of the Farallones sanctuary chief oversees the northern half of the Monterey sanctuary.

San Francisco spent millions replacing its rickety 1930s-era sewage treatment plant with the new Oceanside plant near Ocean Beach, which filters sewage more thoroughly and deposits the treated wastewater through an outfall pipe more than four miles offshore, instead of in the surf line.

The Port of Oakland and other agencies that dredge shipping lanes and berths in the bay use much of the mud and for wetlands restoration projects now — or deposit it near Alcatraz in the bay. Both agencies said this week that they want to see more details in NOAA’s plan before taking a position. A sanctuary designation means no oil drilling, which wasn’t an issue near the bridge anyway — and it doesn’t affect fishing. But it does means that NOAA has more oversight on issues like limiting tourist helicopters and personal watercraft, requiring permits for fireworks shows that could disturb seabirds, tidal energy projects and issuing fines to vessels or cities that pollute the ocean.

“We’re certainly not opposed to it,” said Mary Currie, a spokeswoman for the Golden Gate Bridge District. “It sounds like it won’t impact us much. But we’ve been operating here since 1928, and we want to take a look more closely at the proposal to make sure it won’t affect our maintenance or seismic upgrade work.”

NOAA hopes to have an environmental-impact statement done by next summer, with a final decision by the end of 2013, Brown said.

“It’s a success story. We’ve cleaned up the sewage. The wildlife has come back,” Brown said. “We want to protect this area, to make sure our kids and grandkids will have the opportunity to see the amazing wildlife and incredible views that draw people form around the world.”

Paul Rogers covers resources and environmental issues. Contact him at 408-920-5045. Follow him at Twitter.com/PaulRogersSJMN