On this independence Day weekend, most Americans are probably more worried about the threat of rain on their picnic than about a threat to their fundamental freedoms. Yet our most basic freedom -- the right to free expression of opinion under the First Amendment -- is under increasingly heavy assault these days. And the attack is being encouraged by the Supreme Court.

The Supreme Court has been eroding the First Amendment by slowly and systemically leaving the defense of basic individual rights to state courts and legislatures, which are far more susceptible to pressure from the special interests and other local groups with political clout.

Nowhere is this trend more evident, or more disturbing, than in the continuing abridgment of freedom of the press.

By narrowing the press's protection from libel suits and leaving interpretation of the First Amendment to local judges, the Supreme Court is endangering the existence of the one institution powerful enough to battle the politicians on anything close to an equal footing.

One effect of the Supreme Court's rulings against the press has been an epidemic of libel suits. In Oklahoma, for example, one major Tulsa paper estimates that since 1976 it has spent at least $100,000 on legal paper work to defend itself against 12 libel suits that never even made it to trial.

Last year, a county court jury slapped the Daily Oklahoman, the state's largest-circulation newspaper, with a $1 million libel judgment. The successful plaintiff was George Miskovsky, a candidate for the U.S. Senate who drew less than 2 percent of the vote in the 1978 Democratic primary.

"It is the largest verdict ever attained by a candidate for office," the successful litigant boasted to my reporter Jeff Drumtra. And Miskovsky is apparently correct. Until the Burger Court began its hatchet job on the First Amendment, public officials and political candidates had been fair game for critical press coverage. The interesting thing about Miskovsky's libel suit is that it arose from the newspaper's criticism of a campaign tactic that itself appeared to come perilously close to defamation. Two weeks before the primary, Miskovsky called a press conference to ask Gov. David Boren, the front-runner and eventual winner of both the primary and the Senate seat, four questions:

"Do you know what a homosexual or bisexual is? Are you a homosexual or bisexual? Have you ever been a homosexual or bisexual? Have you ever engaged in homosexual or bisexual activity?"

The Oklahoma press reacted with varying degrees of outrage. Miskovsky, a lawyer, offered no evidence about Boren's sexual proclivities, but said he had heard rumors and was merely asking questions. "I have never charged and still don't charge that Boren is homosexual," he said.

The Daily Oklahoman scolded Miskovsky for what it called his "gutter theatrics," Miskovsky sued.

When most of his case was thrown out by a judge in one county, Miskovsky took it to a different county. "I believed I had a better chance in Cleveland County," he explained. "People there aren't as mesmerized by the Daily Oklahoman."

It was a shrewd maneuver. Members of the jury said later they disliked both Miskovsky and the Daily Oklahoman, so they decided to punish them both by giving the plaintiff "only" $1 million of the $25 million he had sought.

Now Miskovsky is suing the Daily Oklahoman for $50 million -- for reprinting the libelous articles in its coverage of his first lawsuit.

Footnote: Two months ago, I wrote about the plight of the Lake Charles, La., American Press, which had been sued by a local official who had taken umbrage over a critical editorial. The state courts had awarded the plaintiff $175,000 in damages and fees. On June 15, the U.S. Supreme Court refused to hear the case, thus leaving in force a state appeals court ruling against the American Press.