25 Pa. Stat. § 2878
The nominee of each political party for the office of President of the United States shall, within thirty days after his nomination by the National convention of such party, nominate as many persons to be the candidates of his party for the office of presidential elector as the State is then entitled to. If for any reason the nominee of any political party for President of the United States fails or is unable to make the said nominations within the time herein provided, then the nominee for such party for the office of Vice-President of the United States shall, as soon as may be possible after the expiration of thirty days, make the nominations. The names of such nominees, with their residences and postoffice addresses, shall be certified immediately to the Secretary of the Commonwealth by the nominee for the office of President or Vice-President, as the case may be, making the nominations. Vacancies existing after the date of nomination of presidential electors shall be filled by the nominee for the office of President or Vice-President making the original nomination. Nominations made to fill vacancies shall be certified to the Secretary of the Commonwealth in the manner herein provided for in the case of original nominations.
25 P.S. § 2878