At which stage legality check is conducted?

Federal Law on Popular Referendum (March 14, 2014)

Article 13. The request for popular referendum may be filed before the Chambers of the Congress accordingly, under the terms of this Law, as of September 1st of the second year of Administration of each legislature and until September fifteenth of the year before Election day.

Article 28. The following shall apply when the request is submitted by citizens:

I.  After the request  has been received by the Chairman of the Executive Board of the corresponding chamber, he shall have it published in  the Parliamentary Gazette, and shall  report  it and request the Institute to verify, in a thirty-natural days’ period, that it has been subscribed, in a number equal to the two percent of the subscribed in the Voters registration list;

II. In the event that the Institute determines non compliance with the requirement established in Article 35, fraction VIII, numeral 1, paragraph c) of the Constitution, the Chairman of the Executive Board of the corresponding Chamber shall have the report published in the Parliamentary Gazette, report and proceed to file it as a fully and finally concluded case;

III.  In the event that the Institute  determines compliance of the requirement established in fraction I, the Chairman of the Executive Board of the corresponding Chamber shall have the report publish in the Parliamentary Gazette and turn the request to the Supreme Court, together with proposal question of applicants to resolve about its constitutionality within a twenty natural days’ period;

IV. After the application of the Chairman of the Executive Board of the corresponding Chamber has been received to verify the constitutionality of the request for popular referendum, the Supreme Court shall:

a)  Resolve  on the constitutionality of the matter of the popular referendum and review that the question directly rises from the matter of the referendum; that is not biased or contains value judgments; uses neutral, simple and comprehensible language; and produces a categorical answer in the positive or negative sense.

b)  Make, where necessary, the corresponding changes to the question in order to guarantee it is consistent with the matter of the referendum and complies with the criteria stated in the previous paragraph.

c) Notify the corresponding Chamber about its resolution within the following twenty-four hours after issuing it;

V. Should the resolution by the Supreme Court be in the sense of recognizing the constitutionality of the matter, the question in the resolution may not be subjected to later changes by the congress;

VI. In the event that the Supreme Court declared unconstitutionality on the matter of the popular referendum, the Chairman of the Executive Board of the corresponding Chamber shall have the resolution published in the Parliamentary Gazette and report and proceed to file it as a fully and finally concluded case, and

VII. After constitutionality has been declared by the Supreme Court, the Congress through its Executive Boards shall issue the Call, notify the Institute for the corresponding effects and have it published in the Official Gazette of the Federation.

Article 29. The resolutions by the Supreme Court shall be final and unassailable.