Well frankly I think a president could/can/does spy through the NSA, likely spy on anyone they want from a practical not a legal standpoint without a FISA warrant. However all information gained would be inadmissible in court because not obtained under legal evidentiary auspices. FISA warrant = legal spying on a US citizen.  However the court is part of the judicial branch, as a check and balance. So it appears to be that the very process of getting FISA warrant demands its presentation to the court because the FISC appears to be the ultimate authority in granting permission to legally spy.    But I'm sure a president has the capacity to direct the FBI to generate a FISA request that will be submitted to the FISC given that as a unitary executive, the FBI is ultimately under the president's control. And it is likely that some of these judges may have been pressured or complicit by/with the White House.