POP MUSIC REVIEW : Rob & Fab Swim, Don’t Sync, on TV : The discredited duo atone for their Milli Vanilli days by singing their new single live before Arsenio Hall’s adoring audience.

The national capacity for forgiveness is a considerable one. But the sad saga of Rob & Fab may test it.

“Everyone has a chance for a rebirth, to start anew.” This was not a minister talking, or a campaign strategist, but rather the warm-up man on the set of Arsenio Hall’s talk show before taping began Monday, imploring the seemingly reticent audience to put past hurts behind, extend the warm touch of the kindness of strangers and give it up for the erstwhile Milli Vanilli.

“I want you to give Rob & Fab their proper due,” he said, half-ordering, half-begging.

The thing is, to be blessed with this mass grace, you have to persuade the American people you’re properly penitent first. Ross Perot: Really, really sorry he dropped out of the race before . . . surging in the polls again. Woody Allen: Not sorry at all he took up with that young girl . . . new movie bombs.

Rob Pilatus and Fab Morvan: Extremely sorry they scandalously lip-synced to other people’s voices on record and tour . . . embraced by “Arsenio” audience like prodigal sons.

The disgraced duo--who returned their “best new artist” Grammy after the ruse was revealed two years ago--couldn’t have humbled themselves more during the interview with Hall that (wisely) preceded their very first live, it’s-really- their -voices singing appearance. They tossed some blame around, but accepted their fair share.

“We were stupid . . . we won’t make the same mistake twice . . . today we get a second chance and we’re real grateful.” They even thanked Jerry Buss for continuing to give them Lakers tickets in the midst of their shame while everyone else deserted them. Not that they hadn’t deserved to be abandoned, they added in so many repentant, almost groveling words.

Bingo.

The empathetic crowd was there with them , as it were, when Rob & Fab (as they’re now professionally known, the Milli moniker being history) hit the stage to debut their new independent-label single, “We Can Get It On,” along with their new, this-is-really-us voices. So primed was the audience that they half-hoped Caruso would come out of those mouths but would have settled for anything resembling the carrying of a tune.

As it turned out, to break the equation down: Fab did manage to keep his tune aloft, while an obviously nervous Rob fumbled his, though not necessarily fatally.

On the couch with Hall, who is always so supportive of his guests that he makes Larry King look like tough-guy Sam Donaldson, Rob was the chattier of the two and an ideal talk-show guest, self-deprecatingly funny and charming.

Once he started performing, though, Rob adopted the terrified look of the proverbial deer caught in the studio spotlights. Fortunately, his part singing the chorus amounted to a glorified background vocal, while Fab sang the more prominent verses in a fairly confident tone not that far afield from, say, Michael Jackson’s lowest register.

The choreography--trotting around in concentric circles and slapping hands; jerking limbs from side to side in tandem--looked awfully goofy. But then, it looked just as goofy when these two were the beloved of millions.

The mostly tourist crowd seemed taken with the act. “They sang as well as those other people they lip-synced for. Or better,” said Sylvia Cox, visiting from Kentucky, after the taping.

“They were really humbled, they said they’d messed up, and that made me have respect for them,” agreed Dawn Torres of Seattle. “I was ready to have a good time and just laugh at them. But their voices were good--if it was really them.”

Hall, who opened the show with clips of some old Milli Vanilli jokes, waxed enthusiastic afterward.

“That was fly! “ he told the studio audience during the last commercial break. “Just between us 300, if Michael Jackson or Prince did that (stuff), it would be boom , to the moon! . . . It’s just a matter of whether America is gonna forgive. And they forgave Nixon! They forgave me for going out on a date with Sinead O’Connor!”

(We did?)

The real test will come with the single’s release today (on Taj Records) to a wider public that hasn’t had the chance to re-warm to the lads so up-close and personally. If Rob & Fab do make it big again, it may be time for a remake of the movie musical “Singin’ in the Rain”--with a new ending, in which Jean Hagen, exposed for syncing Debbie Reynolds’ voice, confesses all, wins America’s sympathy and, properly chastened, goes on to still greater glory.