"I'm ashamed and I apologise for the bad conduct of some people, to those Spaniards who in good faith voted for the Popular Party (PP)," Rafael Hernando, the party's parliamentary spokesman, told Spanish television in comments posted on his Twitter account.
📺Me avergüenza y pido perdón a los españoles que de buena fe votaron al @PPopular por las malas conductas de algunos. Hemos aprobado más de 70 medidas para que esto jamás vuelva a ocurrir#HernandoAR pic.twitter.com/ejWT6vS1nN— Rafael Hernando (@Rafa_Hernando) January 25, 2018
Hernando is not implicated in the trial currently taking place, which centres around allegations that the PP in Valencia received illegal financing for three election campaigns in 2007 and 2008.
The case has further tainted the conservative party of Prime Minister Mariano Rajoy, which has been hit by several corruption scandals.
On Wednesday at trial, Ricardo Costa, former head of the PP in Valencia, claimed in court that allegations of illegal financing were true.
"It's true that the PP financed itself with dirty money in election campaign acts in 2007," he said, laying the blame on former Valencia president Francisco Camps.
PP officials in the region are suspected of getting business people to pay a company to help organise their campaigns, in order to circumvent a mandatory cap on electoral spending.
Businessmen who have taken the stand in the trial in Spain's National Court, which deals with major criminal cases, have admitted to paying the money.
But they have not said what -- if anything -- they got in exchange for allegedly doing the PP this favour.
The trial is the latest in a series of scandals to hit the PP, contributing to the party losing its absolute majority in 2015.
Rajoy acknowledged that graft issues "hurt us more than the decisions taken in the realm of economic policy" -- a reference to a raft of unpopular austerity measures taken during the economic crisis.
But Hernando said that the PP-led government had attempted to address the issue of corruption with a number of measures.
The government pushed a bill through parliament in 2015 aimed at tightening party accounting rules and stiffening penalties for public workers found guilty of fraud or influence peddling.