The Socialist party of Jose Luis Rodriguez Zapatero, the Spanish prime minister, is hoping to remain part of the governing coalition in Galicia and score an historic victory in the Basque Country.
If the Socialists were to win outright in the Basque election, or lead a resulting coalition, it would be the first time a party whose policy is to keep the region part of Spain would gain power since the area gained broad autonomy in 1979.
Analysts credit the rise in support for the Socialists to the prime minister's attempts 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 has 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 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.