Mexico's ruling Institutional Revolutionary Party (PRI) performed a fast U-turn on Sunday to allow private investment in the oil industry.
Nearly 5,000 PRI members voted unanimously at their national convention to remove policy provisions that for years had barred the entry of private capital into the sector.
Petroleos Mexicanos, or Pemex, is the only company that can carry out oil refining in Mexico.
In their rush to humour big business, the delegates also erased the party's opposition to VAT on food and medicines.
President Enrique Pena Nieto claimed the energy and fiscal reforms were needed to make Mexico more competitive.
"The PRI is seeking to bring the changes Mexico needs," Mr Pena Nieto told a crowd of thousands.
"The PRI is choosing to re-examine and redefine where it stands on the challenges facing the country."
Mr Pena Nieto's intention to open the oil company to more private and foreign investment has set off alarm bells among leftwingers about the privatisation of an enterprise whose nationalisation has always been seen as a source of national pride.
He had previously denied any plans to privatise Pemex.
A meeting of opposition mayors called for protests in mid-March to oppose the new PRI platform.
During 12 years in opposition, PRI members firmly opposed such measures proposed by then-ruling National Action Party, arguing that the country would lose sovereignty by allowing foreign investment in Pemex. They also alleged that taxing food and medicine purchases would severely affect the poor.
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