At the height of a corruption scandal five years ago, Prime Minister Mariano Rajoy of Spain denied his party had operated a slush fund, shortly after his former treasurer was found to have hidden millions in Swiss banks.

Mr. Rajoy also promised full transparency to stop a scandal based on what he called “apocryphal” evidence from further undermining his party. “This is as far as it will go,” the prime minister said at the time in a televised address.

Now, however, the issue of corruption in Mr. Rajoy’s Popular Party has returned with a vengeance to the top of Spain’s political agenda, threatening the survival of his minority conservative government.

On Friday, the main Socialist opposition party presented a parliamentary motion to hold a vote of no-confidence to oust Mr. Rajoy, a day after his party’s former treasurer, Luis Bárcenas, was sentenced to 33 years in prison and fined 44 million euros (about $51.3 million) for benefiting from a kickbacks-for-contracts scheme.