Investigating judge Pablo Ruz of the National Court cleared the way for the trial of the lead suspect in the case: the Popular Party's ex-treasurer Luis Barcenas, a former ally of conservative Prime Minister Mariano Rajoy.
Ruz gathered evidence that the party "from 1990 to at least 2008 drew on various sources of funding outside of the legal economic sphere", his tribunal said in a statement.
"This allowed it to function, at least during the 18 years under investigation, with various systems of accounts which registered deposits and withdrawals of money outside of the official accounts declared" to financial authorities.
Ruz called for Barcenas to stand trial on charges of tax fraud and embezzlement and referred the case to state prosecutors.
The Barcenas scandal erupted in 2013 just as Rajoy's government was fighting to drag the eurozone's fourth-biggest economy out of an economic crisis.
The prime minister admitted he made a mistake in trusting Barcenas but resisted calls to resign.
The affair originated in 2009 as a judicial investigation into alleged kickbacks involving members of the Popular Party and construction companies.
It exploded when leading daily El Pais published copies of account ledgers purportedly showing irregular payments to top party members including Rajoy, its leader since 2004.
In July 2013 Barcenas testified in court that he handed cash to Rajoy.
The prime minister and other senior party members denied receiving any undeclared payments and kicked Barcenas out of the party.
El Mundo newspaper later published what it said were text messages Rajoy sent to Barcenas in which he urged the former treasurer to "be strong" in the face of the scandal.
The judge on Monday also called for five other suspects to stand trial, including another former PP treasurer and members of a construction firm, Unifica.
He dropped charges against 22 other suspects.
The allegations sparked outrage among ordinary Spaniards struggling with spending cuts that Rajoy had imposed during a deep recession.
Spain holds a series of local and regional polls over the coming months followed by a general election around November.
Recent opinion polls have shown corruption to be one of voters' top concerns.
New political parties such as Podemos and Ciudadanos have surged in the polls, campaigning with vows to get rid of what they brand a corrupt political elite.
Rajoy apologised in parliament in October last year for another scandal over an alleged kickback scheme implicating members of his party.
Separately, another Popular Party stalwart, former International Monetary Fund head Rodrigo Rato, is also under investigation by the court over alleged fraud.
"I understand that Spaniards are fed up and outraged," Rajoy told parliament in October.
"This behaviour is especially hurtful when Spaniards have had to endure so many sacrifices to get our country out of the economic crisis."