NEW ORLEANS (AP) _ There's no business like show business, and no one knows it better than the Republican Party. They're camera-ready, color-coordinated and poised for what is billed as the world's largest indoor balloon drop.

One hundred-fifty-thousand of them. They're due Thursday night after George Bush completes his speech - five to seven minutes of reds, whites and blues, freed from their nylon nets by a carefully orchestrated flick of hands.

''Frame that and salute it every morning,'' said an exuberant Bob Johnson, assistant producer of the convention.

President Reagan was feted with 20,000 balloons after his speech Monday night, and another 50,000 hung from the Superdome ceiling to mark the moment Bush won the nomination.

The 150,000 after Bush's speech will be ''stagger-released,'' alternating colors dropping from opposite sides of the arena onto delegates presumably wild with excitement and enthusiasm.

''There's an art to what we do,'' said Sam Kinnard, the site manager for Balloon Art by Treb, a Los Angeles firm that also claims to hold the record for the largest outdoor drop - 1.5 million balloons for United Way of Cleveland two years ago.

''No one is anywhere near'' the company's outdoor record, Kinnard boasted. 'I think there is a group of Japanese who did 400,000.''

While the Cleveland extravaganza was seven months in the making, Kinnard said, the Republican convention was ''almost a last-minute thing'' - three weeks of preparation.

The company brought in 18 Balloon Art members from Philadelphia, New York and Los Angeles to supervise the drops. The Republican National Committee has been providing 50 volunteers for each of three shifts a day to help blow up the balloons.

One woman in the convention production office said she tried to entice the city prison to send volunteers. It didn't work; she and her mother ended up among those enlisted.

This is the way a balloon drop starts: A compressor feeds air into nozzles, volunteers insert the balloons onto the nozzles, inflate them, tie them by hand, then cram them like sausage stuffing into long nylon nets.

The balloon sausages are anchored to the ceiling in rows. The nets are laced so that one tug on a string will release the cargo and send it drifting downward.

For Thursday night, Kinnard has devised an elaborate release scheme. A red sausage on the right side of the Superdome ceiling will be unlaced, followed by a white one and a blue one. Then a red sausage on the left side, followed by white and blue. And back to the right side. Et cetera.

''It'll come off in four stages, which will make the release last anywhere from five to seven minutes,'' Kinnard said.

The timing will have to be exquisite, but the Balloon Art folks consider the convention a piece of cake compared to their record-setting United Way balloon drop in Cleveland.

''The night before, we had a hurricane that came through,'' said Kinnard. ''It was almost a disaster, but we managed to repair the damage and it went of without a hitch.''