President Reagan lobbied across space today, calling for help from orbiting Sen. Jake Garn (R-Utah) in winning Senate approval next week of an aid package for "contra" rebels fighting the Nicaraguan government.

"You're doing a fine job up there, Jake," Reagan told Garn from the Oval Office. "But I could use your help down here right now . . . in arranging assistance for some people fighting for their freedom in Central America. So don't stay up there too long."

"Mr. President, I've missed you, but I'll be back on Tuesday," Garn replied from the space shuttle Discovery. "I'm well aware of the vote on Nicaraguan aid on Tuesday night, and I'll be voting just the way you'd like me to when I get back."

The Discovery crew is scheduled to return to Florida's Kennedy Space Center at 7:16 a.m. EST Friday in what is forecast as almost perfect weather. In addition to Garn, the first congressional observer in space, the crew includes astronauts Karol J. Bobko, Donald E. Williams, Jeffrey A. Hoffman, S. David Griggs and Margaret Rhea Seddon and McDonnell Douglas engineer Charles D. Walker, who made his second space flight to supervise production of an unidentified hormone that cannot be made on Earth.

Even as Discovery's crew prepared for landing, the crew of the shuttle Challenger was going through rehearsal for a scheduled liftoff a week from Monday.

Reagan's chat with Garn was the 14th time in 16 missions that the president had called a shuttle crew in orbit. But it was the first time he has talked with a key member of the Republican-controlled Senate in space. Garn is chairman of the Senate Appropriations subcommittee that overseas the National Aeronautics and Space Administration's budget and is chairman of the Senate Banking Committee.

Reagan did more than enlist Garn's aid on the Nicaragua aid measure; he also put in a playful plug for Garn's help in getting him a ride on the shuttle.

"You know, Jake, maybe in around four years or so, you could use your influence with NASA to get a certain retired politician a ride . . . ", he said.

Garn did not have time to reply, but he more than made up for it when he and his fellow crew members had an inflight news conference with reporters on the ground.

Did Garn get sick?

"Yes, I did . . . . I didn't feel good for two days, but I've sure felt fine since."

On being an astronaut or senator:

"If I were 10 years younger, I would choose the working astronaut over being a senator so fast it would make your head spin."

On the crew's unsuccessful attempt to rescue a stranded Navy Leasat satellite:

"Man is needed in space. You can't do the things we have done up here with unmanned space probes. You need brains; you need minds up here that can think, that are innovative."

How did Garn help on the flight?

"I've been helpful when they needed hands, sewing on the 'fly swatter' [used in the unsuccessful attempt]. It took a little bit of extra strength I was able to add. I've taken photographs. I've done some cooking. And I've tried to be as helpful as I can."

Did Garn enjoy the experience?

"I'd love to show you how I can do everything [Olympics gymnast] Mary Lou Retton can do up here. I even grew an inch and a half . . . .

"Senate life can be mundane and boring. I suppose when I get back, some days when we have those late-night sessions, I'll be saying, 'What in the world are we doing here listening to all this hot air?' "

When the other members of the crew finally got a chance to talk, a few of them waxed almost poetic about their experience in space and their unexpected rescue mission.

"It's a great feeling to rendezvous with a small speck in the sky a hundred miles out in front of you," Bobko said. "It really gives you a feeling of awe to see a big satellite there just a few feet away from your spacecraft."

Physician/astronaut Rhea Seddon said the flight "reminded me of Girl Scout camp. We had bits and pieces of everything out. We were measuring and cutting and pasting and wondering what in the world the thing [the fly swatter, a makeshift rescue apparatus they built] was going to look like when we finished up."

Jeff Hoffman, the crew's astronomer, said he has loved every minute of the flight. "Every time a spacecraft goes through a sunrise or sunset, the Earth below us goes black and the sun is still shining on the orbitor. And we are accompanied by a cloud of ice crystals and it's like trailing a luminous galaxy behind us, leaving hundreds of thousands of particles spinning over and over."