Well before President Clinton kicked off his campaign for the North American Free Trade Agreement this week, local opponents picked Los Angeles Harbor as the starting point for their drive to defeat the pact.

Congresswoman Jane Harman (D-Marina del Rey), a Democrat whose coastal district stretches to San Pedro, announced in a rally at Ports O' Call last month that she opposes the agreement. Accompanying her was Los Angeles City Councilman Rudy Svorinich of San Pedro.

Port executives say NAFTA opponents chose the wrong backdrop. The trade pact, they say, would provide a big boost to the Port of Los Angeles, the West Coast's busiest trading hub.

"You've got to look at the pact from a marketing standpoint-- as a non-competitive, pure shipping issue," said Al Fierstine, the port's director of marketing. "Regardless of what the politicians say it will do, it (will bring) good new business."

Longshoremen's union leaders do not share that optimism. Some of them fear NAFTA would allow shipping companies to shift to Mexico high-paying clerical jobs now based at the port.

But port executives insist that NAFTA will spur the harbor's already fast-growing trade with Mexico. And that, they say, would benefit local transportation businesses such as freight-forwarding companies and trucking firms, as well as their suppliers.

Said Fierstine: "It will have a ripple effect."

Harbor officials base their optimism on a recent surge in port business that they say is the direct result of lowered trade barriers with Mexico.

Since the mid-1980s, when Mexico started to dramatically reduce tariffs and subsidies on a variety of goods, Southern California's exports to Mexico jumped, from $1.6 billion in 1986 to almost $5 billion in 1992, according to the Southern California Assn. of Governments.

Imports, meanwhile, grew at a slightly slower pace, from $1.9 billion in 1986 to $4.9 billion last year.

In one year alone, the port saw its revenue from wharf and dock fees related to Mexican trade climb from $2.5 million in 1991 to $8.8 million last year. Equal, if not greater, jumps could be expected in the years after NAFTA is approved, Fierstine said.

The agreement, which will lift tariffs on goods traded between the U.S., Canada and Mexico over a 15-year period, "will formalize what is already happening," Fierstine said. "What it means is the growth will be at a faster pace than it is today."