California highway officials and many local officials have steadfastly pressed for a 4.2-mile inland bypass around the crumbling cliff-side section of Highway 1. But construction has been blocked in court for three decades by environmentalists who have joined growing grass-roots support for a mile-long tunnel through San Pedro Mountain on the San Mateo County coast.

Given the stubborn tunnel-bypass stalemate, it might be easier to move a mountain than to get people to agree on a Devils Slide solution. But tunnel advocates believe that the federal report made public Thursday shows officials are responding to populist support for a tunnel.

''I think they've come around to this point because there is such local support for a tunnel outside of the environmental groups,'' said Mitch Reid, a member of Pacifica's Tunnel Alternative for Highway 1. ''Devils Slide has been portrayed as environmentalists vs. Caltrans. But it's our (residents') back yard that's most affected by this project.''

An unstable cliff, Devils Slide has routinely crumbled into the Pacific since the road was built in 1937. Torrential rains in January washed out a 170-foot section of the highway linking Half Moon Bay and Pacifica. Commute traffic in coastal communities was paralyzed and coastal businesses suffered as repair crews took five months and $1.4 million to stabilize and reconstruct the roadway.

Opponents say the bypass's deep roadway cuts would scar coastal mountains and pose dangerous fog conditions. Supporters of a bypass say it's better than nothing, and it already has hard-to-get federal funding that could get slashed if locals argue too long. Currently, the tunnel has no federal funding, according to FHA officials.

Federal and state highway officials stressed Thursday that the bypass remains the solution of choice for Devils Slide.

In a review of a Caltrans supplemental environmental study issued earlier this month, the Federal Highway Administration said Caltrans needed to update the bypass's original 1986 environmental analysis, which grew stale during protracted court battles. But federal officials added that because of strong advocacy for a tunnel from engineers, residents and environmentalists, the agency ''commits to address the issue of a tunnel alternative . . . ''

''If there is information out that says a tunnel may be a viable alternative, then we'll re-evaluate it and make a decision,'' said Bill Wong, a senior FHA engineer in Sacramento. ''We don't want to forgo any options at this point. We'll keep our minds open and go from there.''

Yet it will be up to Caltrans, which opposes a tunnel as too costly, to evaluate and present the new assessment, Wong said. ''But if the public has some information that's vital to the project, we'll look at that, too.''

Both sides present conflicting estimates. Caltrans says a two-lane tunnel would cost about $130 million to build and $1 million annually to maintain, while a bypass would run about $80 million. But tunnel advocates have produced a 1993 Caltrans report estimating the tunnel cost at $77 million. They also present private construction firms who estimate a tunnel cost at about $60 million.

Caltrans officials stand behind the bypass project, saying the updated environmental study should be completed by May 1996.

''A lot of people brought up the tunnel issue (in environmental report comments), and so what the feds are saying is, 'Answer their questions,' and that is what we will do,'' said Caltrans spokesman Greg Bayol. ''There's nothing new or shocking about this. The reasons why a tunnel isn't viable haven't changed. We'll just reiterate those reasons.''

But popular support for the tunnel is being felt in local politics. Tunnel advocates have gathered 7,000 signatures of support and raised $7,000 at a sold-out fund-raiser Sunday in coastal Montara. They vow to target elected officials who refuse to explore their project.

In July, the Pacifica City Council withdrew its support of the tunnel and demanded that Caltrans make public its tunnel research. Caltrans inflamed the council's ire by sending five pages on a project that's been rehashed for decades.

Now, the Devils Slide debate is heating up the race for three San Mateo County supervisors seats in the coming March elections. Until recently, a 3-to-2 majority of the board has supported the bypass alternative, fearing any waffling would jeopardize the funding.

Supervisor Mary Griffin, who has been the swing vote in the board's 3-2 support for the bypass, said Thursday that she welcomes further study of a tunnel alternative -- as long as it can get funding.

''I have never said that I would not support a tunnel alternative,'' Griffin said. ''If the Federal Highway Administration asks for a study and it comes out that this tunnel is a viable alternative then the funds aren't in jeopardy.''

Griffin's opponent, Janet Fogarty, has cast herself as the candidate who would ''study the tunnel with an open mind.''

''There are still some unanswered questions about the tunnel, but they're not going to get answered unless someone takes the time and has an open mind to see if it's a reasonable, feasible alternative,'' said Fogarty, a land-use lawyer and a veteran Millbrae council member.

It's failure to study all alternatives that's kept people fighting in court instead of building a highway, she added.

''Let's look for some creative solutions that will accommodate the community and the concerns of the environmentalists, and let's get something built before the next storm shuts down Devils Slide again.''