In a rare interview on Spanish television since leaving power in 2004 and handpicking Rajoy as his successor in the center-right People's Party (PP), Aznar called on the government to cut taxes and create new jobs.
Although credited with bringing Spain's public finances under control, Rajoy has been under fire for failing to reduce a 27-percent unemployment rate and bringing an end to recession.
While declining to say if he would back Rajoy again, Aznar, who ruled from 1996 to 2004 and is widely credited with overseeing Spain's economic boom, said he would be willing to return to frontline politics if circumstances dictated.
"I have never hidden from my duty. I will do my duty (to) my conscience, my party and my country. Have no doubt about that," Aznar said on Antena 3 television when asked his intentions.
This would be a blow for Rajoy, who enjoys a comfortable majority in parliament and has little to fear from a weak socialist opposition but has been challenged in his party for his handling of the economic crisis as well as for his tepid response to corruption scandals involving senior PP members.