Some 4 million Spaniards in the autonomous regions of Galicia and Basque Country were called to vote Sunday in polls that could affect a political stalemate in the capital, Madrid.
Acting Prime Minister Mariano Rajoy is hoping for a victory in his home region of Galicia, where his conservative People's Party has been in power for the last four years.
The nationalist party PNV has been the ruling power in Basque Country since 2012.
If the opposition Socialists, who are blocking the formation of a new central government in Madrid, suffer a bad loss in the regional elections, it could have consequences for party leader Pedro Sanchez.
He has so far rejected a coalition with Rajoy, but may not be able to refuse if he is weakened by this weekend's vote.
Spain has not had a functioning government for almost nine months.
Earlier this month, the parliament rejected Rajoy's latest attempt to win a confidence vote to form a new government, edging the country closer to a third election in a year.