Lindsey Graham, perhaps the Senate's leading hawk on military intervention in Syria, says the most important part of U.S. strategy there is "supporting vetted opposition forces." Bob Corker, ranking Republican on the Senate Foreign Relations Committee, says he is dismayed by "the lack of support we are giving to the vetted moderate opposition." The committee's amendment to the intervention resolution, authored by John McCain, calls for strengthening the "vetted elements of Syrian opposition forces."

In Washington, use of the word "vetting" is usually confined to unknown political candidates and cabinet nominees. So what is this vetting in Syria everyone is talking about? Is the U.S. government requiring opposition fighters to fill out questionnaires? Show photo ID? Hand over bank statements and tax returns?

Whatever it is doing -- a good bit of it is classified -- the Obama administration, along with some supporters on Capitol Hill, claims its vetting can distinguish the good guys from the brutal jihadist killers among the Syrian rebels. But some key members of Congress remain very concerned.

"In places like Syria, vetting can be unreliable and inconsistent," Republican Rep. Michael McCaul, chairman of the House Homeland Security Committee, told me via email recently. "So far, the administration has not made a compelling case that it can differentiate between the factions, or that it even knows the makeup of the factions. The conclusions it has drawn as a result of its vetting are in stark contrast to the briefings I've received, and I remain concerned that a large part of these rebels pose a great threat to our interests."

The true nature of the Syrian rebels has turned into perhaps the pivotal issue in the intervention debate. Among the many question that opponents of intervention have, perhaps the most fundamental is this: Who are we helping?

The question came up when Secretary of State John Kerry recently visited both House and Senate foreign affairs committees. And it became clear that there are stark differences in opinion -- and "opinion" seems the right word -- over who is who in the Syrian opposition.

"Who are the rebel forces?" asked McCaul, who receives classified briefings in his role with the Homeland Security Committee. "Every time I get briefed on this it gets worse and worse, because the majority now of these rebel forces -- and I say majority now -- are radical Islamists pouring in from all over the world to come to Syria for the fight."

Kerry strongly disputed McCaul's question. "I just don't agree that a majority are al-Qaida and the bad guys," he told McCaul. "That's not true. There are about 70,000 to 100,000 oppositionists, about somewhere, maybe 15 percent to 25 percent might be in one group or another who are what we would deem to be bad guys."

McCaul would not accept Kerry's numbers. "The briefings I've received, unless I've gotten different ones or inaccurate briefings, are 50 percent and rising," he said. "These fighters coming globally are not coming in as moderates. They are coming in as jihadists."

McCaul later said he was "stunned" by Kerry's assertion. When it comes to how many Syrian rebels are good and how many are bad, the U.S. government cannot come to agreement with itself.

Kerry explained that he has met several times with opposition leaders in the last year. "They have evolved ... significantly," he said. "Are they where they need to be? Not completely. But they have changed markedly over the course of the last few months."

The rebels, many of whom are militantly Islamist, are now inclusive, Kerry claimed. "At our insistence ... they reached out and expanded significantly their base within Syria. They elected new leadership. They brought in a much broader base of Syrian representation, including women, including minorities, Christians, others."

After Kerry spoke, Deputy National Security Adviser Tony Blinken told Fox News the administration is working with the "moderate opposition" and aims to "make sure that not only our assistance but the assistance of other countries goes to them and stays away from the radical extreme opposition."

In the next moment, Blinken conceded, "It's not a perfect science."

No, it's not. Are violent jihadists one-quarter of the rebel forces, or one-half, or something else? No one seems to know.

The problem intensified with the circulation of a video showing Syrian rebels summarily executing captured government soldiers. There are other such videos around -- the incident was in no way the first example of rebel viciousness -- but the pictures seemed to highlight the dilemma as the intervention debate rages on Capitol Hill. The Syrian civil war is a very ugly thing for the U.S. to be involved in, no matter how much vetting goes on first.

Byron York is chief political correspondent for The Washington Examiner.