WASHINGTON (AP) _ The Supreme Court today let stand a ban on political campaigning from the sidewalks outside post offices across the nation.

The court, without comment, turned down a Connecticut man's free-speech challenge of a federal regulation prohibiting campaigning on Postal Service property.

Frank Longo, described by his lawyers as an anti-establishment politician, sued the Postal Service after a run-in with authorities at the post office in Torrington, Conn., during his 1988 attempt to run for the U.S. Senate as an independent.

To qualify for the ballot, Longo was required to collect about 9,800 signatures on a petition. He decided to seek those signatures outside post offices.

According to court documents, several postal patrons complained that Longo was abusive when they refused to sign his petition.

Longo was shown a copy of the federal regulation and asked to leave. When he refused, he was arrested.

The charge was later dismissed, but Longo did not obtain enough signatures to get his name on the ballot. A former mayor of Bristol and unsuccessful candidate for governor, Longo sued in 1989 so he would be able to use post office sidewalks in future campaigns.

A federal judge ruled that the postal regulation violated Longo's free- speech rights because it was based on the content of his activity and was ''under-inclusive.''

The 2nd U.S. Circuit Court of Appeals, however, reversed the judge's ruling after finding that the regulation was not content-based. The Supreme Court last October told the appeals court to restudy the case.

The justices had just ruled that a Tennessee law prohibiting electioneering within 100 feet of polling places was content-based but valid.

The 2nd Circuit court upheld the campaigning ban again last December.

At issue throughout Longo's legal battle has been whether the area outside a post office is a ''public forum'' where free-speech rights are given enhanced protection.

The appeals court's December ruling said the sidewalk between a post office and its parking lot is not a public forum, and therefore regulations limiting free-speech opportunities need only be reasonable.

The Supreme Court in 1990 that the nation's post offices may ban all soliciting on their property.

But that 5-4 ruling did not resolve whether the sidewalks like those outside the post office in Torrington are a public forum.

The case acted on today is Longo vs. Postal Service, 92-1610.