WASHINGTON — Senator Patrick J. Leahy, chairman of the Senate subcommittee that oversees foreign aid, said Tuesday that he would not support additional military aid to Egypt in the wake of mass death sentences handed out by Egyptian courts this week, adding significant pressure on the Obama administration to shift course.

For months, Senator Rand Paul, Republican of Kentucky, has led a lonely effort to sever American support to one of its most stalwart allies in the Middle East after the military’s overthrow of Egypt’s elected Islamist government. That push appears to be gaining steam.

“I’m not prepared to sign off on the delivery of additional aid for the Egyptian military,” Mr. Leahy, a Vermont Democrat, said on the Senate floor. “I’m not prepared to do that until we see convincing evidence the government is committed to the rule of law.”

On Monday, an Egyptian court sentenced the top spiritual leader of the Muslim Brotherhood along with more than 680 others to death in connection with the killing of a single police officer during a riot last summer. Another court in Cairo banned the activities of a left-leaning protest group on espionage charges.