The legal ignorance underlying the claims by House Intelligence Committee Chairman Devin Nunes (R-Calif.) is easiest to address. Surveilling a target under FISA requires a demonstration, to the special court set up to oversee the law, of "probable cause" that the individual is an "agent of a foreign power." Probable cause, the Supreme Court has explained, requires only a "fair probability" that the asserted facts are true. In instances where a FISA application contains a misstatement or omission, the essential question, the Supreme Court has said, is whether there would have been probable cause without the error. There are many, many decisions from federal courts that address that question, and their answer is almost always yes. Judges know that sources often come with bias or baggage of one sort or another, so they routinely accept and uphold affidavits that generally describe a source's shortcomings without including every detail.
Second, the Nunes memo presents a familiar challenge for the government, albeit from an unfamiliar direction: the congressional oversight committee. News reports indicate that Christopher Steele, the former British secret agent, was a source for the FISA application. Steele was being paid through Fusion GPS by political opponents of President Trump. Based on my familiarity with the FISA process and the people who run it, I have high confidence that they would have provided the court with enough information to meaningfully assess Steele's credibility and the provenance of the other information on which they relied.
One of the main ways to validate a source is to corroborate his reporting. At least some parts of the Steele dossier apparently remain both salacious and unverified. But other parts may well have been verified by independent investigation. And if they were, that verification would strengthen the credibility of the remainder.
To take a fanciful example, imagine that the government had a microphone implanted in Vladimir Putin's tea kettle and overheard him explaining how he expected Trump to do his bidding in return for cash payments. Such information would certainly support Steele's reporting. But if it were revealed, it might cause Putin to discard his kettle and so deprive us of a valuable intelligence source. In short, the price of refuting Nunes may be very high.
Third, the release of the memo imperils the architecture of oversight constructed four decades ago, after the revelations of abuses by intelligence agencies. In response to the Church Committee report, the House and Senate Intelligence Committees were established to deal with the challenge posed by secret intelligence in a democratic society.
The committees serve, in essence, as an informed and trusted proxy for the American people to ensure that U.S. intelligence agencies are not running roughshod over the civil liberties of citizens. For example, in response to the 2013 disclosures by Edward Snowden about the National Security Agency's metadata program, the intelligence committees provided helpful context, alerting their constituents that the collection programs in question had in fact been fully briefed to Congress and approved.
Any system of oversight is going to be subject to occasional strains between agency and overseer, especially when the stakes are so high, secrecy is paramount, and fundamental values are involved. So there have been moments of tension between Congress and the intelligence community over whether the agencies have been adequately forthcoming and otherwise acted appropriately. There have been moments of tension, as well, between Republicans and Democrats on the House and Senate intelligence panels.
But the Nunes memo is different because it so clearly abandons procedural regularity and any pretense of bipartisanship, and because Chairman Nunes has not seriously engaged with the agencies his committee purports to oversee. He has, in effect, both assumed bad faith on the part of the intelligence community, and arguably practiced it himself. That kind of behavior may generate short-term political gains, but it is highly corrosive to the paradigm of intelligence under law that we have used to govern ourselves for the past 40 years.