Analysts credit the rise in support for the Socialists in part to Jose Luis Rodriguez Zapatero, the Socialist prime minister's, bid during his first term to negotiate peace with Eta, the armed Basque separatist group.
Those negotiations eventually failed and the group resumed its attacks.
The polls have raised fears of Eta attacks, blamed for 825 deaths in its 40-year campaign for an independent Basque homeland.
Eta have called the elections undemocratic and told supporters to post blank ballots.
Any parliament resulting from this weekends polls would be a "fascist parliament", Eta said in a statement published in Gara, a pro-independence newspaper, on Friday.
"For those in favour of independence, of sovereignty, the only vote is blank," it said.
No party openly supporting Eta is permitted to participate in the elections.
Two parties have in turn been banned due to alleged links to the Eta's political wing, precipitating Eta's comments on Friday.
Basque separatist parties are said to have the support of about 10 per cent of the local population.
The Nationalist Party has ruled the region for nearly 30 years, and continually suggests that it will pursue independence. It currently heads a coalition that holds a 32-seat majority.
Polls consistently say that the region's citizens are virtually split on whether to have independence or remain with some autonomy.