In the campaign for tomorrow's general election in Spain, the centre-right party led by the two-term Prime Minister, José Maria Aznar, looked to be heading for an easy victory with its popular hard-line stance on terror, backed up by a strong record on the economy.
But the search to determine who was responsible for Thursday's attacks on packed commuter trains in Madrid, which killed 199 people and injured a further 1,430, could have an impact on the outcome.
As the government sought yesterday to determine whether the 10 devastating explosions were the work of the Basque separatist terror group Eta or a cell linked to Osama bin Laden's al-Qa'ida network, editorial writers in heavyweight national media were watching closely.
The Spanish daily newspaper El Mundo - a staunch supporter of the ruling Popular Party (PP) throughout the campaign - said it was important to bring all the evidence to light so that Spaniards can go to the polls knowing who is the author of the massacre.
The party, which is being led into the elections by Mr Aznar's handpicked successor, Mariano Rajoy, had campaigned on its hardline stance against Eta, and for a constitutional model that guarantees Spain's territorial integrity. But if the attacks prove to be the work of Islamic militants, they could be viewed as the price for the government's backing of the US-led war in Iraq.
"If the hell unleashed which burned the whole of Madrid on Thursday is the result of Islamic fundamentalism, we must look at Spain's role in the Iraq war: an involvement which our citizens rejected, a personal decision by the Prime Minister beyond the wishes of the majority," said the commentator Antonio Gala.
Mr Aznar faced massive street protests at home when he backed President George Bush's plans to invade Iraq at the UN Security Council - of which Spain was a temporary member - before the war a year ago.
He later sent 1,300 troops, including a brigade of legionnaires from Central America and the Caribbean, for peacekeeping duties in the southern Iraqi city of Diwaniya.
The pro-US policy has already appeared to place Spaniards in jeopardy. Spanish interests were singled out by Islamic militants in Morocco during a series of co-ordinated suicide attacks that killed 40 people in Casablanca in May last year. The majority of casualties were diners at the Casa de Espana social club in the city, which was frequented by diplomats and business visitors.
Mr Rajoy, who has held four ministerial posts under Mr Aznar, headed the government's response to the sinking of the Prestige oil tanker in November 2002. In preparations for the election - the ninth since Spain hammered out its post-Franco constitution in 1978 - he has set out a stall offering continuity with the government's policies on terrorism.
Mr Rajoy's rival is not so vulnerable to the possible revelation of an al-Qa'ida link to the bombings. Key to the campaign waged by the Spanish Socialist Workers' Party leader, José Luis Rodriguez Zapatero, who is seeking to return the party to power after eight years of opposition, has been the separation of Spain's foreign and security policies.
Should he win tomorrow's poll, Mr Zapatero has promised to bring home the troops engaged in peace-keeping in Iraq when their mandate comes to an end on 30 June. Mr Zapatero, 43, who took over the weary and divided party in 2000, has also pledged to maintain a tough line against Eta - a commitment questioned by Mr Rajoy.
In the most recent poll, taken on 8 March, the socialists had narrowed the PP's lead to 4.5 percentage points from 6.2 percentage points in an earlier survey. The PP were seen as winning between 168 and 173 seats, short of the 176 needed for an absolute majority. Polls over the weekend said it would win between 162 and 172.
Should forensic investigations and security force inquiries reveal today that the attacks were carried out by Eta - a charge denied by the group yesterday - then the PP looks set to seal its lead.Reuse content