Previous chief justices Warren Burger and William Rehnquist, both conservatives, also named judges to the intelligence court, but their selections were more ideologically diverse, the analysis revealed. Under those justices, 66 percent of the selections were Republican appointees, while only 39 percent had prior jobs in the executive branch.

Timothy Edgar, who has advised both the Bush and Obama administrations on civil liberties related to intelligence issues, said, "If the court is seen as skewed or biased, politically or ideologically, it will lose credibility."

Sen. Richard Blumenthal, D-Conn., says the study shows "people with responsibility for national security ought to be very concerned about the impression and appearance, if not the reality, of bias -- for favoring the executive branch in its applications for warrants and other actions."

Blumenthal has proposed the chief judges from each of the 12 major appeals courts pick a district judge for the surveillance court. The chief justice would continue to name an appellate panel, but six other Supreme Court justices would have to sign off on the selections.

Rep. Adam Schiff, D-Calif., has introduced a bill that would give the president the power to name the surveillance court judges, with the approval of the Senate.