Spain's governing Popular Party said at the weekend that it will investigate the financial activities of former treasurer Luis Barcenas.
A court said he had amassed an unexplained €22 million (£18.5m) in Swiss accounts.
That sparked street protests in Madrid and Barcelona on Friday and Saturday.
Popular Party spokeswoman Maria Dolores de Cospedal said: "We will have to go over everything we have done to prove to all Spaniards that our hands are clean."
On Friday, Deputy Prime Minister Soraya Saenz de Santamaria denied knowing anything about the money or newspaper reports that Mr Barcenas secretly gave large sums of it to party members.
The same day, Ms Cospedal denied that the party had distributed "black cash bonuses" from the construction industry to some of its top leaders.
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