Sen. Gary Hart (D-Colo.) was high in the hills of Bel Air last night on the patio of a $4 million house with the lights of the city flickering far below when one of those lines that campaign managers have nightmares about slipped out.

Hart had hopscotched across the country, spending six hours on an airplane, and his wife, Lee, had met him at the airport.

"The deal is we campaign separately," the tired candidate told gay activists assembled for a fund-raising reception. "That's the bad news. The good news for her is she campaigns in California and I campaign in New Jersey."

Lee Hart broke in to say that she had held a koala bear while campaigning in California.

"I won't tell you what I got to hold--samples from a toxic waste dump," her husband snickered.

The lines got a big laugh. This, after all, is affluent California, proud and sassy, almost a nation unto itself.

But it didn't take Hart aides long to realize that while a New Jersey joke might play well here, it certainly would not in self-conscious New Jersey.

Press secretary Kathy Bushkin quickly put out word that the candidate hadn't intended any disparagement of New Jersey. He was simply complaining about the long, coast-to-coast plane rides he was having to make, she said.

Hart adviser Frank Mankiewicz said he was "counting on" Hart's rival, Walter F. Mondale, to "jump on it, and drive it into the ground and make more of it than is there."

Mondale, campaigning in northern California, proclaimed "I love New Jersey" and said Hart owed the Garden State an apology.

What the remark illustrated more than anything else is the dangers of a cross-country campaign as the three Democratic presidential campaigners hop back and forth between New Jersey and California, the two most populous states holding primaries June 5.

It is particularly clear in Hart's case. There is almost a Hart East and a Hart West campaign. Each operation has its own strategy, media adviser, commercials, kingpins and issues that it wants to stress.

For example, Hart spent most of Thursday afternoon filming television commercials in New Jersey, using the Meadowlands sports complex as a backdrop. Those commercials are being prepared by Ray Strother, his longtime media adviser.

Today Hart spent the afternoon filming California commercials, using Union Square in the heart of San Francisco as a backdrop. William Zimmerman, Hart's new California media adviser, is preparing those ads.

According to advisers, the California commercials will attempt to restore the sense of momentum Hart enjoyed around the New Hampshire primary in February, by being upbeat, positive and futuristic.

The New Jersey ads, meanwhile, will try to identify the candidate with economic progress in that state.

"The California campaign is sort of an entity unto itself," said Strother. "They have their own strategy, we have ours."

Inevitably there are conflicting pressures from both states, and personality disputes. Strother, for example, learned of Zimmerman's California role by reading a newspaper story. It upset him, and he has made his displeasure clear in conversation with friends.

"We try to be one happy family, but sometimes that's not easy," said another Hart adviser.