Boeing and Lockheed Martin, the aerospace giants with decades of experience working on America’s space program, will happily sell rockets to carry astronauts into space, but the companies are leery about taking a leading role in President Obama’s vision for a revamped National Aeronautics and Space Administration.

The prospect of NASA relying on smaller companies — unproven upstarts in the view of critics — could create yet another hurdle in convincing an already skeptical Congress of the idea of relying on commercial companies to provide taxi transportation to the International Space Station.

“I don’t think there is a business case for us,” John Karas, vice president and general manager of human spaceflight at Lockheed Martin, said about space taxis.

Publicly, Boeing has been enthusiastic. “We have a vested interest in the International Space Station,” Brewster H. Shaw, vice president and general manager of Boeing’s NASA Systems business unit, said in February after NASA awarded the company $18 million to draw up a preliminary design of a capsule that could serve as the basis of a commercial crew system. “We want to see I.S.S. live up to its potential by having a robust logistics for the delivery of cargo and crew.”