Rep. Adam Schiff is one of the leading Democrats warning that the visa waiver program leaves the United States more vulnerable to terrorist threats than programs to resettle Syrian refugees. (AP Photo/Cliff Owen, File)

As many Republicans urge a post-Paris moratorium on Syrian refugees, a band of leading lawmakers – including many Democrats – are warning that the United States visa waiver program poses the greater threat to security.

“I would tell you, from a threat standpoint, I’m probably more concerned with the visa waiver program today,” said Sen. Richard Burr (R-N.C.), chairman of the Senate Select Committee on Intelligence. “Were I in Europe already, and I wanted to go the United States, and were I not on a watch list or a no-fly list and I wanted to get there, the likelihood is I would use the visa waiver program before I would try to pawn myself off as a refugee.”

Much of the political debate following the Paris attacks has focused on whether to suspend or restrict the influx of Syrian immigrants into the U.S. seeking refuge from civil war in their country.

But some lawmakers on Tuesday warned that the much bigger problem was the visa waiver program, which was enacted in 1986 and brings almost 20 million people to the U.S. every year.

Under the program — which covers 38 countries, 30 of which are in Europe, including France, Belgium and Germany — citizens of waiver countries can enter the U.S. with minimal screening. That makes it much harder to identify and weed out European nationals seeking to enter the U.S. after traveling to ISIS training camps in other nations and then returning to Europe.

“Europe has a much more massive number of people who’ve come back” from jihadist training centers than does the U.S., said Rep. Adam Schiff (D-Calif.), following a classified briefing with Homeland Security Secretary Jeh Johnson and FBI Director James Comey on Tuesday. “And they’ve come back to visa waiver countries.”

Schiff, the ranking member of the House Permanent Select Committee on Intelligence, said that the U.S. has a more limited ability to track Europeans returning from ISIS strongholds than Americans who do the same, due in part to restrictions on information sharing. But the waiver program doesn’t take that into account.

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Travelers taking advantage of the visa waiver program must have a valid passport, register electronically before traveling, and their stays are limited to 90 days. The tourist and business travelers eligible for the program are still screened, but there is no requirement for a face-to-face interview before entering the country.

That’s great news for those taking advantage of the program. But it makes the system far more porous to the type of terrorists that carried out the Paris attacks, lawmakers warned, than the refugee resettlement program. Several of the Paris perpetrators were French or Belgian nationals.

“Our ability to keep track of those people is much more limited,” Schiff said. “And frankly, they pose a far greater risk, if the past is any indication, than people who are vetted in a year-and-a-half to two-year long refugee process. Those folks can come with a visa waiver.”

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Lawmakers raised concerns about the program earlier this year, in the aftermath of the Charlie Hebdo attacks in Paris. But renewed scrutiny of the waiver system isn’t likely to sit well with all lawmakers.

The waiver program is considered a driver of tourism. Lawmakers from states that depend on tourists to fuel their economies have pushed for expansion of the program in recent years.

“Visa waiver is very important to our ability to have our guests, foreign guests, come here and enjoy our attractions, which are both natural and unnatural,” said Florida Democratic Sen. Bill Nelson, whose state boasts everything from the Everglades to Disney World.

Nelson said he was open to a discussion about changing the program, but warned against being “too quick to judge.”

“Let’s first evaluate everything that has happened,” Nelson said. “But thus far I don’t think we know of any terrorist that has come in to the U.S. on visa waiver.”

Lawmakers also anticipated pushback from companies that rely on the visa waiver program to facilitate short-term business travel if they start to reexamine the program.

“I think it’s going to be difficult to do because the business sector likes the visa waiver program – and you get into that conflict,” predicted Sen. Dianne Feinstein (D-Calif.), ranking member on the Senate’s Intelligence committee. But she too believes the waiver program is more problematic than the refugee resettlement system.

“I happen to agree that the visa waiver is the easiest because somebody that goes to fight in France, a visa waiver country, comes back to his country and then he decides he wants to come to California,” Feinstein said. “So we need to look at that and explore what the options are, and how to make prudent changes that don’t impact our economy dramatically but do offer a significant measure of protection.”

But what those changes might be, no one is yet sure.

Senate Foreign Relations Committee ranking member Ben Cardin (D-Md.) said Democrats would be committed to taking “every action” necessary to “not only contain but destroy” ISIS and protect the U.S.

“That includes looking at the refugee resettlement program, it includes looking at visitors that come in through the visa waiver program, it includes better intelligence information,” Cardin said. “Whether they’re adequate or not, they need to be reevaluated in light of what happened in Paris.”

Mike DeBonis contributed to this report.