The minority Socialist-led government needs the backing of several parties, including Catalonia's separatists, to steer its spending bill through an initial vote Wednesday. The separatists, and right-wing parties, have said they will vote against the budget.
Sanchez tweeted that "the right-wing and the separatists will vote against a budget that helps social causes. They both want the same thing: a Catalonia that is divided and a Spain that is divided." If the bill survives, it would then enter a second phase of debate and amendments to specific areas of spending.
The opening of the parliamentary budget debate coincides with the start of a politically-charged trial at Spain's Supreme Court, where 12 Catalan separatist leaders face charges for their role in a 2017 secession attempt.
Spain's taxation minister says the Spanish government won't yield to pressure from Catalan separatist parties to open talks on the northeastern region's self-determination in exchange for support for the country's 2019 budget.
Maria Jesus Montero, who is the minister in charge of the national spending plan, says the socialist government "will not give in to any blackmail by anybody."
Prime Minister Pedro Sanchez could be forced to call an early election if the separatists, whose support brought the Socialists to power last year, don't change their current position of voting against the budget on Wednesday.
Montero has said as the key parliamentary debate opened that "under no circumstance will we admit that the right to self-determination in Catalonia appears in any talking points."
The former leader of Catalonia's independence movement who fled from Spain has called for 12 separatists starting trial in Madrid to be absolved for their alleged crimes.
Carles Puigdemont has said in Berlin that the "trial that has started in Madrid this morning is a test for the whole Spanish judiciary system, therefore it is a stress test for the Spanish democracy."
He adds that "I trust, however, that the Spanish state will take advantage of this chance to issue the correct sentence, which is absolution."
Puigdemont successfully avoided extradition from Germany when a German court refused to send him back to Spain on charges of rebellion last year. He has been living in Belgium since fleeing Spain in Oct. 2017 after defying warnings by Spanish authorities and leading a secession bid that Spanish courts say violated the Constitution.
The dozen high-profile separatists whose trial at Spain's Supreme Court began Tuesday face charges that include rebellion.
Protesters have blocked highways in Spain's northeastern region of Catalonia, as the trial of separatist leaders gets underway in the country's Supreme Court in Madrid.
Tuesday's protests, which have also led authorities to close off some main roads in the Catalan capital of Barcelona, were timed with the start of arguably Spain's most consequential trial in four decades of democracy.
Twelve defendants are being tried for their roles in pushing ahead with a unilateral independence declaration based on the results of a secession referendum that ignored a constitutional ban. Some face decades in prison if they are found guilty of rebellion.
The proceedings are being broadcast live on television.