Article 63 Political Constitution of the United Mexican States

The Chambers may not open their sessions nor exercise their office without the attendance, in each one of them, of more than half of the total number of their members; but those present of one and the other shall meet on the day designated by the law and compel the absent ones to attend within the following thirty days, with the warning that if they do not do so it shall be understood by that sole fact that they do not accept their office, calling then the substitutes, who must present themselves within an equal term, and if they also do not do so, the position shall be declared vacant. Both the vacancies of deputies and senators of the Congress of the Union that arise at the beginning of the legislature, and those that occur during its exercise, shall be filled: the vacancy of deputies and senators of the Congress of the Union by the principle of relative majority, the respective Chamber shall call extraordinary elections in accordance with the provisions of section IV of Article 77 of this Constitution; the vacancy of members of the Chamber of Deputies elected by the principle of proportional representation shall be filled by the slate of candidates of the same party that follows in the order of the respective regional list, after having been assigned the deputies that may have corresponded to it; the vacancy of members of the Senate elected by the principle of proportional representation shall be filled by that slate of candidates of the same party that follows in the order of the national list, after having been assigned the senators that may have corresponded to it; and the vacancy of members of the Senate elected by the principle of first minority shall be filled by the slate of candidates of the same party that for the state (federal entity) in question has registered in second place of the corresponding list.

It is also understood that the deputies or senators who are absent ten consecutive days, without justified cause or without prior leave of the president of their respective Chamber, of which the latter shall be informed, renounce to attend until the immediate period, calling forthwith the substitutes.

If there is no quorum to install any of the Chambers or for them to exercise their functions once installed, the substitutes shall be summoned immediately to present themselves as soon as possible to perform their office, while the thirty days of which there is talk above run.

Those who, having been elected deputies or senators, do not present themselves, without justified cause in the judgment of the respective Chamber, to perform the office within the term indicated in the first paragraph of this article, shall incur responsibility and shall become liable to the sanctions that the law indicates. The National Political Parties that, having nominated candidates in an election for deputies or senators, agree that their members who are elected do not present themselves to perform their functions shall also incur responsibility, which the same law shall sanction.