Perhaps "Speak Up, America" is caused by a virus. That would be one excuse for George Schlatter's listless abomination, a person-on-the-street variety show returning to NBC tonight for an early fall start that is not in any way promising. Death could never be considered premature for a clinker like this.

Two editions of the show aired last April, and when its return was announced, NBC spokesmen insisted that "Speak Up" would be cleaned-up and the program's host, former child evangelist Marjoe Gortner, would be toned down. On the premiere tonight, at 9 on Channel 4, Gortner concedes that he may have been "a little bit too flamboyant" and "a little enthusiastic" and says, "I'm going to try to hold it down."

Then he launches into a tirade similar to the rabble-rousing he did on the previous programs, a histrionic display that belies NBC President Fred Silverman's recent promise that "we've taken all that out of the show."

The format is divided into segments and gimmicks purportedly giving vox to the populi, but the sequence are so severely edited and spliced together that the program becomes a medley of opinionated one-liners --- punk schlock. Watching the program is like being trapped on a street corner with an expostulating wino. Since remarks opposing, say, the ERA ("I would never cross a bridge that a bunch of women built," says a woman to the the audience's cheers) are balanced off with remarks defending it, "Speak Up" soon degenerates into a classic of nil.

Gortner, less personable than the average used-car salesman, is joined by cohost Jayne Kennedy, late of CBS Sports, an a gangly, gosh-almighty comedienne named Ronda Bates, who managed to trick President Carter into saying "Speak up, America" for a blurt of tape seen early in the program.

The show also makes use of the Warner-Amex two-way cable QUBE system in Columbus, Ohio, asking viewers there to vote on questions like "Who should have to register for the draft?" The tabulations should pop up instantly, but because most of tonight's program was taped in advance, the poll results are confined to an updated insert not available for preview.

Basketball player Magic Johnson is also seen in the program's most appealing, least lapel-grabbing sequence, as he talks to kids in his home town and gives them the opportunity to "tell American anything." Says one young man: "You need a black president."

Other real people intervied hither and you do brighten up the program on occasion. Asked "Who shot J.r.?", the evil character on "Dallas," one young man responds, "My Aunt Didi did it. She shoots everybody." Foolish questions like "who is the sexist man in America?" are intermingled with questions about women's rights and unemployment in the auto industry. NBC announced last week that, alas, a segment devoted to a Mr. Tush Contest" had been postponed to a later date.

Perhaps "Speak Up, America" could be written off as harmless froth, but some people in television say they find it "frightening," a kissing cousing to the Howard Beale demagoguery revue conjured by Paddy Chayefsky in the movie "Network." Told that NBC News personnel objects to the program and get "hysterical" when the subject comes up, Silverman has said, "They're not hysterical. That's a very strong word. There've been some questions."

And there should be.But high rancor over the program may be overkill, since its own scampering dullness should be enough to consign it to oblivion in fairly short order. It could be said to suffer from the same disease that afflicted "the late Mr. Loopner" on "Saturday Night Live": It was born without a spine.