There is no reason why trading by Members of Congress or their staff members would be considered “exempt” from the federal securities laws, including the insider trading prohibitions.
Members of Congress and legislative staffers owe fiduciary-like duties of trust and confidence to a host of persons including the citizen-investors, the federal government, other members of Congress, and government officials outside of Congress. Based on these duties of entrustment, congressional officials engage in deception, and therefore violate Rule 10b-5, if they trade securities on the basis of material nonpublic information obtained through congressional service.
Additional Reading:
USA Today: Congressional Insider Trading Is Illegal
Business Insider: Congressional Insider Trading Is Illegal
etc.