Congressional Democrats are convinced that the President has broken the law and trampled the Constitution. Some intellectually challenged Republicans just haven't made up their minds and are waiting for the Senate Judiciary hearings to conclude before they announce their positions.
Let me help the Republicans (as the Democrats are absolutely hopeless when it comes to the Constitution and National Security).
Article II, Section 2 states: "The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States."
The Federalist Papers and Court precedents from the beginning of this republic have held that the President acting as Commander in Chief has the inherent power to defend the nation against foreign threats. Since before WWI, that power has been understood to encompass electronic surveillance.