G.R. No. L-4067 - IN RE: Mercado vs. Lacuesta

Prior to May 1, 2009, the province of Bulacan was represented in Congress by four legislative districts.
The First Legislative District comprised Malolos City (under RA 8754, § 57) and the municipalities of Hagonoy, Calumpit, Pulilan, and Paombong.

In 2007, Congress filed House Bill No. 3162 (later House Bill No. 3693) and Senate Bill No. 1986 seeking to create a lone congressional district for Malolos City.
RA 9591 lapsed into law on May 1, 2009, amending Malolos City’s charter to establish it as a separate legislative district.

The 2007 NSO Census placed Malolos City’s population at 223,069.
Congress relied on an undated NSO Region III certification projecting a 2010 population of 254,030 based on a 3.78% annual growth rate (1995–2000).
Dispute arose over the legal effect, authority, and accuracy of this projection for meeting the 250,000‐person constitutional threshold.

Petitioners—taxpayers, registered voters, and residents of Malolos City—filed an original action for prohibition to declare RA 9591 unconstitutional.
They contended that Malolos City did not meet the 250,000 minimum population requirement under Section 5(3), Article VI of the 1987 Constitution and Section 3 of its Ordinance.
The OSG, on behalf of the Comelec, argued that Congress’s reliance on projected population figures was a non‐justiciable legislative determination.

Whether Malolos City had an actual or legally valid projected population of at least 250,000 to merit its own legislative district under Section 5(3), Article VI of the 1987 Constitution and Section 3 of the Ordinance.
Whether the NSO Region III certification based on demographic projections—without NSCB declaration and without certification by the NSO Administrator or designated officer—had legal effect.
Whether the Court may review Congress’s choice to rely on population projections in creating or reapportioning legislative districts.