The PP's regional head, Carlos Iturgaiz, said ETA was trying to intimidate voters.
Manuel Jimenez Abad, the head of the Popular Party (PP) in Aragon, was shot repeatedly in the head in the city of Zaragoza.
Both the ruling Basque National Party (BNP) and the PP have suspended Monday's scheduled campaign and condemned the attack.
The head of the Basque Government, Juan Jose Ibarretxe of the BNP, described the killing as barbaric.
"ETA must know that, despite differences, all political parties respect the right to life, and that is where we are confronting them," he said.
The PP's regional head, Carlos Iturgaiz, said ETA was trying to intimidate voters.
"Basques have got to go and vote on May 13 and tell ETA that they are not wanted, that they have to disappear," he told state television.
Shock
Mr Abad, 53, was killed while on his way to a football match.
Reports say paramedics tried to revive him but failed.
Television pictures showed his body lying by the side of the road and covered in blood, while friends and relatives cried and hugged each other at the scene of the shooting.
"There are no words to describe these moments... it's unbelievable," Gustavo Alcalde, the Popular Party's regional co-ordinator in Aragon, told Spanish state radio.
"As far I know he hadn't received any threats and he obviously did not have a bodyguard," he said.
Regional elections
Officials say the attack bears the hallmark of ETA.
"Three shots were heard in what looks like an assassination attempt by ETA," a spokesman for the Spanish Government in Zaragoza said.
Police recovered several shells from a 9 mm pistol, which they said was the preferred weapon of the separatist group.
Elections for the Basque regional parliament are due to be held next Sunday.
The Popular Party, led by Prime Minister Jose Maria Aznar, is seeking to oust the Basque Nationalist Party for the first time in two decades.
Since ETA ended its ceasefire in December 1999 it has killed 29 people.
The group is fighting for an independent Basque nation.