The secretary general of Spain's governing Popular Party, Mariano Rajoy, says the Basque plan for autonomy is a betrayal of the state that will cause the destruction of the constitution.
By Danny Wood
The Basque regional government on Saturday officially approved the plan, which proposes more independence from Spain.
Spain's political divisions are becoming more apparent as the national government prepares an official response to the plan.
The plan has support among many Basques
Drawn up by the Basque leader, Juan Jose Ibarretxe, the text of the proposed political statutes for the Basque country calls for a referendum to decide whether the region should become a sovereign state.
The issue is causing considerable tension across Spain, especially in the Basque regional parliament.
After an ugly slanging match, a Popular Party deputy was expelled from the house for calling opposition members the heirs of ETA, the armed Basque separatist group.
The Popular Party executive is meeting this week to look at how to oppose the plan by all means possible.
The rhetoric coming from both sides of the debate is very heated.
Prime Minister Jose Maria Aznar says the Basque government's proposal for greater autonomy legitimises the terrorism of ETA and has no chance of succeeding.
Mariano Rajoy, his successor as Popular Party leader, says it is a moral imperative to stop the proposal.
But supporters of the plan claim it could end ETA violence and Juan Jose Ibarretxe denies he wants full independence from Spain.
The next critical phase starts on 4 November with the Basque parliament expected to debate and then approve the proposal, preparing the way for a referendum.