1987 Constitution of the Philippines
Section 17. The Senate and the House of Representatives shall each have an Electoral Tribunal which shall be the sole judge of all contests relating to the election, returns, and qualifications of their respective Members. Each Electoral Tribunal shall be composed of nine Members, three of whom shall be Justices of the Supreme Court to be designated by the Chief Justice, and the remaining six shall be Members of the Senate or the House of Representatives, as the case may be, who shall be chosen on the basis of proportional representation from the political parties and the parties or organizations registered under the party-list system represented therein. The senior Justice in the Electoral Tribunal shall be its Chairman.