But last week, as it happens, C-Span did contact the speaker’s office to have it take down a different clip from her blog — one shot by C-Span’s cameras at a House Science and Technology Committee hearing on global warming where Ms. Pelosi testified, Mr. Daly said. (The blog has substituted material filmed by the committee’s cameras, he said.)

C-Span, a private nonprofit company financed by the cable and satellite affiliates that carry its programming, says that over more than 25 years of operating it has consistently asserted its copyright to any material it shoots with its own cameras. But that message can get lost.

“We are structurally burdened, in terms of people’s perception, because we are the only network that has such a big chunk of public domain material,” said Bruce Collins, the corporate vice president and general counsel of C-Span. He estimated that 5 to 15 percent of C-Span’s programming is from the House and Senate floor, and thus publicly available.

“It is perfectly understandable to me that people would be confused,” he said. “They say, ‘When a congressman says something on the floor it is public domain, but he walks down the street to a committee hearing or give a speech and it is not public domain?’ ”

The issue is of recent vintage for C-Span. In May, C-Span said that it had for first time asserted its copyright against a video-clip site, ordering YouTube to take down copies of Stephen Colbert’s pointed speech in front of President Bush at the White House Correspondents’ Association dinner. Clips of the speech had been viewed 2.7 million times on YouTube in the 48 hours before it was taken down.