MADRID (AP) — A high-stakes trial beginning Monday is bringing back a spotlight on the illegal kickbacks funding scheme that for years has haunted Spain's Popular Party, casting a long shadow on the country's main conservative party despite its pledges of regeneration.

The proceedings at the National Court in Madrid are looking into whether the renovation of the party's headquarters in the Spanish capital more than a decade ago was paid for with an alleged slush fund largely built on bribes.

The case is the latest in a judicial saga that began when allegations of the party's decades-long shadowy accounting first emerged in 2013.

The Popular Party, or PP, which has been a dominating presence over the past four decades of democracy in Spain, has already paid a high political price for corruption scandals by its members or elected officials.

In mid-2018, former party chief Mariano Rajoy was ousted as prime minister in a no-confidence vote after a court ruled that the party had benefitted from a scandalous kickback scheme.

Luis Bárcenas, the party's long-time treasurer, is currently serving a 29-year prison term for that case. On Monday, together with four other defendants, he takes the stand again in the new trial.

The former accountant has admitted to the crimes and threatened to reveal more details that could be damaging for former Popular Party members.