Puerto Rico Constitution
Article III - The Legislature
Section 8.

Section 8. The term of office of Senators and Representatives shall begin on the second day of January immediately following the date of the general election in which they shall have been elected. If, prior to the fifteen months immediately preceding the date of the next general election, a vacancy occurs in the office of Senator or Representative for a district, the Governor shall call a special election in said district within thirty days following the date on which the vacancy occurs. This election shall be held not later than ninety days after the call, and the person elected shall hold office for the rest of the unexpired term of his predecessor. When said vacancy occurs during a legislative session, or when the Legislative Assembly or the Senate has been called for a date prior to the certification of the results of the special election, the presiding officer of the appropriate house shall fill said vacancy by appointing the person recommended by the central committee of the political party of which his predecessor in office was a member. Such person shall hold the office until certification of the election of the candidate who was elected. When the vacancy occurs within fifteen months prior to a general election, or when it occurs in the office of a Senator at Large or a Representative at Large, the presiding officer of the appropriate house shall fill it, upon the recommendation of the political party of which the previous holder of the office was a member, by appointing a person selected in the same manner as that in which his predecessor was selected. A vacancy in the office of a Senator at Large or a Representative at Large elected as an independent candidate shall be filled by an election in all districts.