Support for the previous winner, the conservative People's Party (PP), has collapsed amid a corruption scandal. Its main opponent, the Socialist party, has rocketed to the top of the polls after seizing the prime minister's job last year.
One recent poll showed almost half of voters - 40% - had yet to make up their minds.
The Socialist and Podemos alliance of Prime Minister Pedro Sánchez's existing government needed the Basque and Catalan nationalists to support it.
But the Catalan pro-independence parties were partly responsible for the government's collapse after they pulled their support in February; and the crisis over the failed Catalan independence bid has made the nationalists hugely unpopular in much of Spain - making negotiations with them tricky.
The PP (led by Pablo Casado) and Ciudadanos (Albert Rivera) will probably need the support of Vox - but this is seen as unlikely.
Despite widespread concerns about unemployment - which remains high in Spain compared to its European neighbours - it has barely been discussed by politicians.
"The campaign is going to remain around identity issues, and particularly around the Catalan issue... it seems that the economy is not, any more, the completely fundamental issue," Prof Teruel says.
Before the election, Mr Sánchez had been negotiating with Catalan parties to support his budget. But those talks broke down amid a public backlash over him meeting them, partly stoked by Vox's fervent opposition to any concessions on independence.
Despite the problems, Mr Sánchez's time as leader has seen support for his party surge - at the expense of coalition partner Podemos.
Meanwhile, a corruption scandal involving the PP last year toppled its leader and saw support plummet - leaving plenty of votes to fight for among a fragmented right.
Such an arrangement is precisely what happened in the regional government of Andalusia - where the PP, Ciudadanos, and Vox formed a right-wing regional government for the first time in 36 years.
"It should be noted that they do not display openly any Francoist imagery," he said. "They are not making gross mistakes.