The announcement of the telescope’s potential demise came as part of a draft budget for NASA and other agencies, including the Commerce and Justice Departments. In all, the committee proposed lopping $1.6 billion off NASA’s current budget, which is $18.4 billion for 2011. The Obama administration had originally requested $18.7 billion for NASA.

Astronomers reacted with immediate dismay, fearing that the death of the Webb telescope could have the same dire impact on American astronomy that killing the Superconducting Supercollider, a giant particle accelerator in Texas, did in 1993 for American physics, sending leadership abroad.

Canceling the Webb telescope would “have a profound impact on astrophysics far into the future, threatening U.S. leadership in space science,” said Matt Mountain, director of the Space Telescope Science Institute in Baltimore, which would run the new telescope. “This is particularly disappointing at a time when the nation is struggling to inspire students to take up science and engineering,” he added.

Tod R. Lauer, an astronomer at the National Optical Astronomy Observatory in Tucson, echoed his view. “This would be an unmitigated disaster for cosmology,” he said. “After two decades of pushing the Hubble to its limits, which has revolutionized astronomy, the next step would be to pack up and give up. The Hubble is just good enough to see what we’re missing at the start of time.”

The Webb telescope, he said, “would bring it home in full living color.”

The Appropriation Committee’s proposal was the opening act in what is likely to be a long political drama, in which the Senate will eventually have a say. The measure is expected to be approved Thursday by the subcommittee in charge of NASA and the other agencies, according to Jennifer Hing, a spokeswoman for the committee.