MANILA — Allies of President Rodrigo Duterte appear to have swept the elections for the Philippine Senate, according to unofficial results on Tuesday, giving him a stronger grip on the one legislative chamber that had shown some degree of independence from his authoritarian rule.

With more than 95 percent of the vote counted, candidates backed by Mr. Duterte looked likely to win all 12 of the seats in the 24-member Senate that were up for election in Monday’s voting. If that is confirmed in the coming days by the Commission on Elections, then a small opposition bloc in the Senate that had managed to thwart some of the president’s agenda will become substantially weaker.

One by one, candidates in an opposition alliance known as Otso Diretso, meaning Straight Eight, threw in the towel on Tuesday, and a spokesman for Mr. Duterte essentially declared victory.

“The victory of the administration candidates and the shutout of the Otso Diretso candidates send a strong message that our people yearn for stability and continuity of the genuine reforms that the administration started,” the spokesman, Salvador Panelo, said in a statement. He said the new Senate would help in “crafting the president’s legislative agenda.”