In a potential setback for the government, a federal judge on Friday blocked the introduction of certain evidence that prosecutors wanted to use to support their case that Senator Robert Menendez of New Jersey accepted bribes in exchange for approving billions of dollars in aid to Egypt.

The judge’s order, which comes two weeks into Mr. Menendez’s corruption trial in Manhattan, could undermine prosecutors’ ability to prove certain elements of the multifaceted bribery charges against the senator.

The ruling rests on protections afforded to members of Congress under the Constitution’s “speech or debate” clause, which bars the government from citing specific legislative actions in seeking to prove a federal lawmaker committed a crime.

The U.S. attorney’s office for the Southern District of New York has said it intended to sidestep discussion of official legislative acts and focus instead on promises it says preceded Mr. Menendez’s votes and congressional actions.

The judge, Sidney H. Stein of Federal District Court, ruled in March that although Mr. Menendez’s performance of a legislative act was protected conduct, “his promise to do the same is not.”