The Court held that the determination of compliance with constitutional requirements for creating legislative districts, including the authoritativeness and reliability of population indicators, is a justiciable question subject to judicial review, and that population certifications for such purposes must adhere to standards ensuring reliability, such as those outlined in Executive Order No. 135.

Republic Act No. 9591 was enacted, creating a legislative district for the City of Malolos. This law was challenged, leading to a Supreme Court Decision on January 25, 2010, which presumably found the law unconstitutional. The respondent, COMELEC, filed a motion for reconsideration of this decision, arguing primarily that Congress's reliance on certain population data was a non-justiciable political question.

• Congress enacted RA 9591 creating a legislative district for Malolos City, relying on various population indicators.
• One such indicator was a Certification by Alberto N. Miranda, Region III Director of the National Statistics Office (NSO), projecting Malolos City's population to be 254,030 by 2010, using a 3.78% growth rate between 1995 and 2000.
• Other indicators cited by COMELEC, allegedly used by Congress, included the 2007 Census of Population - PMS 3 Progress Enumeration Report (showing Malolos population at 223,069, later adjusted), a Certification from Malolos City's Water District dated July 31, 2008 (population 281,413), and a Certification from the Liga ng Barangay dated August 22, 2008 (population 258,229).
• The Court found that Miranda's Certification did not comply with Executive Order No. 135 (EO 135) as it was not based on official NSCB demographic projections, was not for the middle of the year, and Miranda was not the NSO Administrator or his designated certifying officer.
• Using Miranda's own growth rate assumption (3.78%) and the official 2007 census population of Malolos City (223,069), the projected population as of August 1, 2010, would only be 249,333, below the 250,000 constitutional threshold.
• The 2007 Census - PMS 3 report contained preliminary data, subsequently adjusted, and was deemed unreliable.
• Certifications from the Malolos Water District and Liga ng Barangay were found not authoritative under EO 135, as these entities are not authorized to conduct population censuses for official purposes.
• The creation of a legislative district for Malolos City by RA 9591 resulted in the town of Bulacan being geographically isolated from the rest of the geographic mass of the former First Legislative District.

• (Inferred from the Court's affirmation of its prior decision) Petitioners Victorino B. Aldaba, et al. argued that RA 9591 was unconstitutional for failing to meet the population requirement of 250,000 inhabitants for a city to have its own legislative district, as mandated by Section 5(3), Article VI of the 1987 Constitution.
• (Inferred) Petitioners likely argued that RA 9591 violated the constitutional requirement that each legislative district shall comprise, as far as practicable, contiguous, compact, and adjacent territory.

• Respondent COMELEC, in its Motion for Reconsideration, argued that Congress's reliance on the Certification of Alberto N. Miranda and other population indicators (2007 Census PMS 3, Malolos Water District cert., Liga ng Barangay cert.) in enacting RA 9591 was non-justiciable.
• COMELEC contended that the Court should not inquire into the authoritativeness and reliability of the population indicators used by Congress.
• COMELEC argued that it was impracticable for Congress to create a district that included the municipality of Bulacan with Malolos City to satisfy the contiguity requirement because Malolos City lies at the center of the First Legislative District.

• Whether the Court should reconsider its January 25, 2010 Decision finding RA 9591 unconstitutional.
• Whether the reliability and authoritativeness of population indicators used by Congress in creating a legislative district are non-justiciable political questions.
• Whether the population indicators relied upon by Congress for enacting RA 9591 (Miranda's NSO projection, 2007 Census PMS 3, Malolos Water District certification, Liga ng Barangay certification) were reliable and authoritative under prevailing standards, particularly EO 135.
• Whether Malolos City met the constitutional population requirement of 250,000 inhabitants to be entitled to its own legislative district before the May 10, 2010 elections.
• Whether RA 9591, by carving Malolos City from the First Legislative District and leaving the town of Bulacan isolated, complied with the constitutional requirement that legislative districts be contiguous, compact, and adjacent.

• The Supreme Court denied COMELEC's Motion for Reconsideration with finality.
• The Court reiterated that the constitutionality of legislative apportionment acts is a judicial question, and subsidiary questions impacting constitutionality, such as compliance with the population requirement under Section 5(3), Article VI of the Constitution, are justiciable. The Court has a duty to determine if there has been grave abuse of discretion by any branch of government.
• The population indicators used by Congress were found unreliable and non-authoritative. Miranda's NSO certification failed to meet EO 135's requirements regarding official NSCB projections, mid-year estimates, and issuance by the NSO Administrator or designated officer. The 2007 Census PMS 3 report was preliminary, and certifications from the Water District and Liga ng Barangay were not from entities authorized to conduct official population censuses under EO 135.
• Malolos City did not meet the 250,000 population threshold. Even using Miranda's own growth rate, the projected population for August 2010 was 249,333. Furthermore, Malolos City was not shown to have breached the 250,000 mark before the May 10, 2010 elections, as required by Section 3 of the Ordinance appended to the 1987 Constitution.
• RA 9591 violated the constitutional requirement that legislative districts comprise, as far as practicable, contiguous, compact, and adjacent territory, because it isolated the town of Bulacan from the rest of its district. The Court stated that including the municipality of Bulacan in Malolos City's legislative district was a practicable alternative to comply with constitutional mandates.

• Judicial Review — The power of the courts to determine the constitutionality of acts of other branches of government. Applied here to assert the Court's authority to review the constitutionality of RA 9591, an apportionment law, and to inquire into the reliability of population data used by Congress, rejecting the claim of non-justiciability.
• Political Question Doctrine (Limits of) — The principle that certain questions are to be decided by the political branches of government, not the judiciary. The Court held that while apportionment involves policy, its compliance with explicit constitutional limitations (like population minimums) is not a political question insulated from judicial review, especially under the expanded certiorari jurisdiction (Art. VIII, Sec. 1 of the Constitution).
• Constitutional Requirements for Legislative Apportionment (Art. VI, Sec. 5(3) of the Constitution) — This provision mandates that a city must have at least 250,000 inhabitants to be entitled to at least one representative, and that each legislative district shall comprise, as far as practicable, contiguous, compact, and adjacent territory. The Court found RA 9591 violated both the population and contiguity requirements.
• Authoritative Population Data (Executive Order No. 135) — EO 135 establishes procedures for providing population data for official purposes, requiring certifications for intercensal years to be based on official NSCB demographic projections, be as of the middle of the year, and issued by the NSO Administrator or designated officer. The Court used EO 135 as the standard to assess the reliability of population data used for RA 9591 and found the data wanting.
• Presumption of Regularity in Performance of Official Functions — The principle that official duty has been regularly performed. The Court invoked this in relation to the NSO's census processes, stating that absent convincing proof of data "sloping," the NSO enjoys this presumption.
• Statutory Construction (Analogy) — The Court noted it is not alien to adopt standards from a parallel statute (like EO 135) to fill gaps in the law where there's no express prohibition, especially when the underlying principles are similar (reliability of population data for creating legislative districts vs. converting LGUs).

• "The constitutionality of a legislative apportionment act is a judicial question, and not one which the court cannot consider on the ground that it is a political question."
• "To deny the Court the exercise of its judicial review power over RA 9591 is to contend that this Court has no power 'to determine whether or not there has been a grave abuse of discretion amounting to lack or excess of jurisdiction on the part of any branch or instrumentality of the Government,' a duty mandated under Section 1, Article VIII of the Constitution."
• "The alleged circumstance that this statute improves the present set-up constitutes no excuse for approving a transgression of constitutional limitations, because the end does not justify the means."
• "Each legislative district shall 'comprise, as far as practicable, contiguous, compact, and adjacent territory.'"

• Macias v. COMELEC, No. L-18684, 14 September 1961, 3 SCRA 1 — Cited to support the principle that the constitutionality of legislative apportionment acts is a judicial question, not a political one, and that the Court can inquire into their constitutionality.
• Navarro v. Ermita, G.R. No. 180050, 10 February 2010 — Referenced as a recent instance where the Court, exercising judicial review, struck down a law creating a province for non-compliance with population and land mass requirements under relevant legislation.
• Menzon v. Petilla, 274 Phil. 523 (1991) — Cited as an example where the Court applied statutory provisions by analogy to resolve a legal question in the absence of an express prohibition, supporting the application of EO 135 standards to RA 9591.

• 1987 Constitution, Article VI, Section 5(3) — Requires cities with at least 250,000 inhabitants to have at least one representative and mandates that legislative districts be contiguous, compact, and adjacent. Central to the Court's ruling on the unconstitutionality of RA 9591 due to non-compliance with population and contiguity.
• 1987 Constitution, Article VIII, Section 1 — Defines judicial power, including the duty to determine grave abuse of discretion by any government branch. Invoked to assert the Court's power of judicial review over RA 9591.
• Ordinance appended to the 1987 Constitution, Section 3 — States that "any city whose population may hereafter increase to more than two hundred fifty thousand shall be entitled in the immediately following election to at least one Member." Applied to emphasize that Malolos City needed to meet the population requirement before the 2010 elections.
• Executive Order No. 135 (EO 135) — Provides guidelines for the National Statistics Office (NSO) in issuing population certifications. Used as the standard to assess the reliability and authoritativeness of the population data Congress used for RA 9591. Specifically, Section 6(e) was cited regarding LGU-conducted censuses.
• Republic Act No. 7160 (Local Government Code), Section 7 — Relates to the creation and conversion of local government units, requiring compliance with income, population, and land mass indicators to be attested by DOF, NSO, and LMB respectively. Referenced to show Congress itself mandates NSO certification for LGU creation/conversion, supporting the use of similar stringent standards for legislative districting.
• Republic Act No. 9591 (RA 9591) — The law creating the legislative district of Malolos City, which was the subject of the case and ultimately found unconstitutional by the Court, with this resolution denying the MR on that finding.

• Puno, C.J., Carpio Morales, Brion, Del Castillo, Villarama, Jr., Perez, and Mendoza, JJ. — Concurred with the main opinion of Justice Carpio.

• Abad, J. — Adopted his dissent on the main opinion. (The resolution notes that Corona, Nachura, Peralta, and Bersamin, JJ., join this dissent).
• Velasco, Jr., J. — Took no part due to relationship to a party.