"You don't have our trust, nor do you have our support," Antonio Hernando, the Socialists' parliamentary spokesman, told Rajoy as he addressed lawmakers.
In a taste of things to come, Hernando blasted the acting prime minister's first term track record in a bitter debate held before Rajoy submitted himself to a preliminary, symbolic parliamentary confidence vote.
As expected, Rajoy lost this vote and he will go through a second, final and decisive vote at the weekend which he will likely win thanks to a decision by the Socialists to abstain.
The Socialists have defended this decision by saying they are helping unblock the political impasse in Spain, which has remained without a fully-functioning government for 10 months after two inconclusive elections.
"There is no reason to maintain the political blockage and take Spaniards to new elections," Hernando said.
"Our abstention on Saturday will allow you to form a government, but it is not support for your government or your policies," he told Rajoy.
Support will be in short supply when Rajoy takes power next week at the head of a minority government, a far cry from 2011 when his Popular Party won an absolute majority.
With just 137 seats out of 350 in parliament, his party will face huge opposition and Thursday's bitter debate reflected this as Rajoy came out fighting and his rivals criticised him.
Pablo Iglesias, the head of far-left Podemos, which aspires to replace the Socialists as the main opposition force, also warned his own party would not "fall into line."
"We are not a left-wing force that fits the mould... we want a different way of doing things and want to change things," he said.
Rajoy, meanwhile, called on opposition parties to try to agree on crucial measures such as the 2017 budget, as Spain works to reduce its deficit under EU scrutiny.
"Demonising your enemy is not credible, it doesn't work," he told lawmakers. "Not having a government is as bad as having a government that can't govern," he said.
"Without a budget, by not complying with our agreements with the European Union, we run the serious risk that Spain will experience a sterile four-year term."