Just after the start of campaigning for the 17 April elections in the troubled Basque region, the Constitutional Court said on Friday it had rejected an appeal from the new grouping Aukera Guztiak (AG) against a ban handed down by the Supreme Court.
AG is linked to armed separatist group ETA and outlawed party Batasuna.
Pro-independence party Batasuna won 10% of the Basque vote in the last regional elections in 2001 but was banned in 2002 and 2003 on the grounds it was an integral part of ETA.
ETA has killed some 850 people in a four-decade-old campaign for independence from Spain and France.
After AG emerged ahead in regional polls this year, Spain's central government asked the Supreme Court to bar its candidates, saying they were instruments of ETA and Batasuna.
AG has denied ties to either group and Batasuna has always denied being part of ETA.
While the courts are cracking down on politicians they say are linked to separatist violence, a police campaign against ETA has brought dozens of arrests in France and Spain and the group has not carried out a fatal attack in nearly two years.
Moderate Basque Nationalist Party Premier Juan Jose Ibarretxe opposed banning AG from the vote but the courts' decision could channel more votes in his direction.
The elections were called a month early as Ibarretxe sought voter support for his plan for near-independence - a plan the Spanish government and main opposition call a violation of the Spanish constitution.