The controversy over FBI surveillance of Carter Page, who advised Donald Trump on foreign policy during his presidential campaign, tends to obscure how easy it is to get permission for a wiretap of a suspected foreign agent. Trump and his allies say the FBI's warrant application relied on information from former British intelligence agent Christopher Steele without making it clear to the Foreign Intelligence Surveillance Court (FISC) that his research had been funded by Hillary Clinton's campaign and the Democratic National Committee. But given the modest legal requirements for wiretapping in foreign intelligence cases, it seems likely that the FBI could have obtained a warrant without using Steele at all.
To monitor the communications of a "U.S. person" (a citizen, legal permanent resident, or U.S. corporation) under the Foreign Intelligence Surveillance Act (FISA), the FBI needs probable cause to believe the target is an agent of a foreign power. In Page's case, the relevant definition of a foreign agent would be someone who "knowingly engages in clandestine intelligence gathering activities for or on behalf of a foreign power," or who knowingly assists such activities, when they "involve or may involve" violating a criminal statute. When a U.S. person is the target, a warrant cannot be issued "solely upon the basis of activities protected by the first amendment."
Unlike when it seeks a warrant in an ordinary criminal case, the FBI did not need to show probable cause to believe that Page had broken the law, only that he was an agent of a foreign power, which might (or might not) involve illegal espionage. The New York Times says the FBI had to show Page was "probably an agent of a foreign power." But if probably means "more likely than not" (i.e., supported by "a preponderance of the evidence"), that is incorrect. Probable cause, which the Supreme Court has described as "a fair probability," is supposed to be stronger than a hunch or a reasonable suspicion, but it is weaker than a preponderance of the evidence, the standard that applies to verdicts in civil cases. To put it another way, the FBI could get a warrant for an American who was probably not a foreign agent.
How hard is it to show the sort of probable cause the FBI needs under FISA? Not very, judging by the bureau's track record before FISC judges, who almost never reject its warrant applications. The FBI's defenders argue that its success rate reflects careful preparation, which requires approval by senior Justice Department officials and often includes consulting with the court before filing. But in the end, Syracuse University law professor William Banks says, the government's burden is not very demanding. "Carter Page was doing business in Russia, talking to Russian diplomats who may have been involved in intelligence activities in the United States," Banks told the Times. "Game over. The standards are incredibly open-ended."