The Popular Party wants to prevent any candidates from Bildu running in the municipal elections that have links to Basque terrorist group ETA, El Pais reported.
The government has not yet received all the reports on Bildu from security forces, but officials said there is sufficient documentation suggesting Batasuna, the outlawed former political wing of ETA, is behind Bildu.
Bildu was created in response to the ban on nationalist group Sortu by the Supreme Court on indications it would form ETA's new political wing.
Bildu, which includes members of Sortu, plans to develop independent candidates without any arrests or links to other nationalist parties, but who are easily recognizable to the public.
Congress is expected to authorize the attorney general's office to act against the 258 candidacies put forth by Bildu.
Municipal elections are to be held in the Northern Basque country May 22.