Seeking to steer the eurozone's fourth-biggest economy out of an economic crisis, the 58-year-old has refused to resign despite allegations that he took secret payments from his party.
With a strong parliamentary majority that has enabled him to push through tough economic reforms, he also benefits from Spain's political culture, experts say.
"In Spain there is no tradition of resigning. Resignation is seen as an extreme political weakness," said Ferran Requejo, a political scientist at Barcelona's Pompeu Fabra University.
"There have been practically no examples of it happening" since 1978 when the current constitution came into force after the end of the Franco dictatorship, he added.
"The basis for this is simply the lack of a democratic tradition in the Spanish state. Spain has only been a democracy for a few years."
Pressure increased on Rajoy over the past week with fresh indications that he may have received illegal payments from a slush fund run by his Popular Party's former treasurer, Luis Bárcenas.
But despite Bárcenas testifying in court that Rajoy received €25,000 ($33,000) in cash in 2010, Spaniards are not seriously expecting their leader to quit.
Relatively few people turned out to protest in the streets at the latest allegations, compared to the mass rallies of recent years by demonstrators angry at economic hardship and high unemployment.
Rajoy looks set to win political redemption — a moral outcome seen by some as typical in Roman Catholic Spain.
"It has a lot to do with Catholic culture. One is never guilty. The guilty one is always someone else," said Fernando Vallespin, a political scientist at Madrid Autonomous University.
"That is not so clear in Protestant cultures where people more naturally take responsibility."
Spanish voters have up to now been relatively indulgent of scandals, experts say.
"In the last local elections, people who had faced charges ran and of those, 70 percent got elected," said Jesus Lizcano, president of the Spain branch of anti-corruption watchdog Transparency International.
"That gives you an idea of how tolerant and disinterested citizens in Spain were about corruption."
The absolute majority enjoyed by Rajoy's party since his landslide election victory in 2011 has blocked calls for him to face questions in parliament over the scandal.
Opposition Socialist leader Alfredo Perez Rubalcaba has threatened to table a motion of censure against Rajoy if he does not appear.
"In any other democracy in the European Union, a political crisis of such gravity would be discussed and resolved here in the seat of popular sovereignty," he told parliament .
The rigid structure of Spain's political parties meanwhile makes an internal rebellion against the Prime Minister unlikely, observers say.
"It is most probable that he will not resign and will try to resist until the last moment, despite the worsening situation in Spain," said Requejo.
Some say however that Spanish politics may face changes as a previously permissive electorate grows weary at the lack of accountability.
"Spanish politics needs purging to break away from this crisis and disaffection with politics," said Vallespin, who would not rule out Rajoy resigning eventually.
"The constitutional consensus needs to be renewed. This scandal could herald the start of that renewal."
The Socialists themselves and even the royal family have been implicated in other recent corruption scandals, impacting on public opinion.
"There used to be an attitude that someone who broke the law and made money from doing so was seen as clever," Requejo said. "But that is changing."