Thursday's bombs are expected to prompt Spain's voters to turn out in their millions for elections on Sunday.
Revelations about al-Qaeda could further confuse an already complex election
"One thing no-one will dispute is that we are now likely to see a massive turnout," said Charles Powell, senior European affairs analyst at the Madrid-based think tank the Elcano Royal Institute.
"People are angry and upset, and they will demonstrate that by voting. It was going to be high anyway, but now I'd estimate it will be over 80%," he told BBC News Online.
Professor Juan Pablo Fusi, historian at Madrid University, concurred.
"These bombs are likely to increase turnout - to show that they agree with the government that the only way to defeat terrorism is through democracy, by voting," he told BBC News Online.
There is less consensus on which party will benefit.
The most recent polls suggested a narrowing lead for the ruling Popular Party (PP), now led by Mariano Rajoy, the hand-picked successor of Prime Minister Jose Maria Aznar.
It was estimated to have a lead of 4.5 percentage points over the opposition Spanish Socialist Workers' Party (PSOE), led by Jose Luis Rodriguez Zapatero.
But that poll was taken three days before Thursday's attacks, which killed nearly 200 people aboard packed rush-hour trains in Madrid.
And the news of the arrest of three Moroccans and two Indians, and that a videotape has been found in which al-Qaeda purportedly claims responsibility for the attacks, could muddy the waters even further.
It had generally been accepted that if Eta was to blame the ruling party would benefit because of its uncompromising stance against the Basque separatists.
"The obvious emotional interpretation is this will make people back the party with the toughest line against them," politics professor Josu Mezo told Reuters news agency.
However, though the involvement of Islamists in the bombings is far from proven, the BBC's Katya Adler in Madrid says that increasing indications of this involvement could cost the government dearly.
"The key point is evidence," said Professor Fusi.
"If it is al-Qaeda, many people may establish a connection with Spain's participation in the invasion of Iraq and vote against the government."
But another factor could see a significant swing to the Socialists, says Mr Powell.
"If al-Qaeda is found to be involved, it could induce young left-wingers - who opposed the war in Iraq, but tend to abstain at the moment because of dissatisfaction with the left-wing parties - to go out and vote... and that could tilt the balance."
For other analysts, prediction is impossible in the face of these attacks, the worst carnage on Spanish soil since the 1936-39 civil war.
"My thoughts are with the dead now, I can't begin to think about the electoral consequences," analyst Juan Diez told Reuters.
"We've never had anything as terrible as this, it's impossible to say how the Spanish people will react. It's guesswork."