Prime Minister José María AznarOnce ridiculed as a colourless technocrat, Aznar has achieved unrivalled political stature in Spain but will not be on the ballot, having decided not to seek a third term.
In 1996, Aznar steered the right into power for the first time since the death of dictator Gen Francisco Franco in 1975.
He was re-elected in 2000 with an absolute majority, allowing his centre-right Popular Party to form a government and pass laws without help from other parties.
Internationally, Aznar is likely to be best remembered for his un- wavering support of US President George W. Bush in the run-up to the war in Iraq despite a public outcry and mass street demonstrations across Spain.
Domestically, he helped revitalise the economy and his government said a clampdown on Basque separatist group Eta had driven it to the brink of collapse. But if Thursday's attack was carried out by Eta then it would be their bloodiest ever.
Aznar has also alienated regional political parties, who might be needed in the event Aznar's hand-picked successor, Mariano Rajoy, needs a coalition to govern.
Popular Party candidate
Mariano Rajoy
Rajoy (48), has held four different cabinet posts plus the position of deputy prime minister, making him eminently well qualified to govern.
Rajoy says his greatest challenge was leading the government's response to Spain's worst ever environmental disaster, an oil tanker spill off the coast of his native Galicia in November 2002.
Rajoy's lengthy public record was built on an image of competence and he has served the centre-right Popular Party without being one of its visionaries. Commentators routinely suggest the public knows little about Rajoy as a person, except that he loves Cuban cigars, cycling and that he supports at least four different football clubs.
Rajoy came up through the ranks in the northern Galician region, starting his public career as property registrar at 23 and elected mayor of the town of Alama in 1981. In recent years he has directed the PP's successful national election campaigns.
While Public Administration Minister in 1996, he married Viria Fernandez and they have a four-year-old son. Rajoy has also held the posts of interior and education minister, as well as Aznar's chief of staff.
Socialist candidate
José Luis Rodriguez Zapatero
Zapatero, 43, took over a party in tatters in July 2000 with the challenge of returning the party to the glory days of former Prime Minister Felipe Gonzalez, who governed for nearly 14 years before being defeated in 1996 in the wake of corruption scandals.
Zapatero, who has been in Congress since 1986, took the top party post after the ruling Popular Party won again with an absolute majority. Zapatero worked closely with the government to sign an agreement to jointly oppose Eta and not use anti-terrorist policies for partisan gain. He later emerged as a more feisty opposition leader by supporting a one-day general strike, criticising the government for its handling of the massive oil spill off Galicia, and opposing the war in Iraq. Zapatero has pledged to bring home the 1,300 troops in Iraq when their mandate ends on June 30th if he wins.
He also has pledged only to form a government if the Socialists win the most votes, raising questions of how he would react if the Popular Party wins the most votes but is unable to form a government.
Zapatero is married and has two daughters.