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A key surveillance program is set to expire at the end of the year, and even its supporters aren't sure it will be renewed. It is Section 702 of the Foreign Intelligence Surveillance Act, and it's been brought up in congressional investigations of Russia's meddling in the U.S. presidential election. NPR's David Welna reports on why some lawmakers think it's a problem.

DAVID WELNA, BYLINE: How important are the spying powers in Section 702 of the Foreign Intelligence Surveillance Act? I put that question to Raj De, who was until two years ago the National Security Agency's general counsel.

RAJ DE: Section 702 is probably one of the most if not the most valuable surveillance authority for the national security community today.

WELNA: That's because U.S. law generally requires time-consuming court orders for any surveillance involving Americans. But under Section 702, that's no longer necessarily the case. Elizabeth Goitein as an expert on that statute at the Brennan Center for Justice.

ELIZABETH GOITEIN: Section 702 essentially legalized President Bush's warrantless wiretapping program. It allows the government acting inside the United States to collect communications between foreign targets overseas and Americans without getting a FISA warrant, which is what was required previously when an American was on one end of the conversation.

WELNA: There still is a requirement to mask the names of American persons or entities in surveillance records. Even so, at a House Judiciary Committee hearing last month on reauthorizing section 702, Goitein testified that Americans emails or phone conversations collected under that statute could be used against them in a criminal case. Republican Ted Poe was indignant.

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TED POE: They get the information on the American, and then they file criminal charges. And all of that is done without a search warrant under the Fourth Amendment to the Constitution of the United States against that American citizen, correct?

GOITEIN: That's correct.

POE: And I think that is illegal and a violation of the Constitution and abuse of power by our government on Americans.

WELNA: As an example of such alleged abuse, Idaho Republican Raul Labrador pointed to recent news leaks about Michael Flynn's phone conversations with the Russian ambassador, leaks that doomed Flynn's brief stint as President Trump's national security adviser.

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RAUL LABRADOR: For me, it had a chilling effect - that I thought my political opponents could use my personal information that they maybe gathered in some private communication against me in the future. That should be quite terrifying to anybody, whether you're a Republican or a Democrat.

WELNA: Such misgivings have supporters of extending the Section 702 program alarm. At the House Intelligence Committees only open hearing on Russian election meddling, Florida Republican Tom Rooney warned NSA Director Mike Rogers that intelligence leaks about Americans fueled opposition to renewing Section 702.

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TOM ROONEY: It's really going to hurt the people on this committee and you all in the intelligence community. When we try to retain this tool this year, in trying to convince some of our colleagues that this is really important for national security, when somebody in the intelligence community says, you know what; to hell with it; I'm going to release this person's name because I'm going to get something out of it, we're all going to be hurt by that if we can't reauthorize this tool. Do you agree with that?

MIKE ROGERS: Yes, Sir.

WELNA: At that same hearing, South Carolina Republican Trey Gowdy pointed out the surveillance of Michael Flynn because the actual target was the Russian ambassador was most likely done under a different surveillance statute.

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TREY GOWDY: But in the eyes of people watching, it is the U.S. government officials leaking the name of a U.S. citizen. And if it can happen here, it may happen there. Trust me. You and I both want to see it reauthorized. It is in jeopardy if we don't get this resolved.

WELNA: For other lawmakers, the real issue is how many Americans' communications get swept up annually in the 702 program.

RON WYDEN: The intelligence leadership has stonewalled on this issue for more than five years.

WELNA: That's Oregon Democratic Senator Ron Wyden. At Dan Coats' confirmation hearing to be director of National Intelligence, Wyden tried extracting a promise from him to come up with an estimate.

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WYDEN: Will you commit to getting this number to this committee and the public before reauthorization?

DAN COATS: Yes, I do. I've got to do everything I can to work with Admiral Rogers and NSA to get you that number. I've been told it's an extremely complex process for a number of reasons.

WELNA: Coats said intelligence agencies might have trouble tallying up all the Americans whose communications had been scooped up under 702 in what's officially known as incidental collection. But the Brennan Center's Goitein believes there's a more cynical reason why there's been no estimate.

GOITEIN: Because the number's going to be enormous at least if it's honestly discerned and reported. If we learned that it was actually tens of millions or more of Americans' communications, we would understand that the term incidental here is somewhat meaningless.

WELNA: Even without that number, skepticism about Section 702 is clearly growing on both sides of the political aisle. David Welna, NPR News, Washington.