Until recently, Mariano Rajoy, 48, was a little known politician. But he quickly earned a reputation within the ruling Popular Party (PP) for absolute discretion and loyalty to his boss, Aznar.
His credibility was enhanced by the PP’s re-election campaign in 2000 which won the party a clear majority.
Rajoy has acted as Aznar's right-hand man in recent political crises, willing to handle the worst tasks in the public spotlight.
He quietly bore the brunt of dealing with Spain's worst-ever environmental disaster amid huge public outcry after the sinking of the oil tanker Prestige in November 2002.
He then took the role of justifying Spain's backing for the US-led war on Iraq in the face of strong public opposition.
Rajoy was elected to the Galician regional government in 1986 and worked his way up the political ladder from there.
Spanish elections are expected in March 2004 and Aznar has long said he will not stand for a third term in general elections.
He will propose a successor to meetings of the PP's ruling bodies next Monday and Tuesday where they are expected to ratify his choice.