But with little more than six weeks to go, the poll of polls showed the far-right Vox party also gaining sharply with a 12.1 per cent vote share.
Vox currently holds no seats in the Spanish parliament and an outcome similar to the poll projection would be regarded as a significant victory for the party, which stands on an anti-immigrant platform.
The snap election was called in February after Mr Sanchez’s budget was voted down.
The vote, the third in under four years, is a further sign of an increasingly fragmented political scene, with Catalonia‘s independence push dominating recent debate.
Mr Sanchez took office after ousting his predecessor, Mariano Rajoy, in June last year, when he won a no-confidence vote triggered by a damaging corruption conviction affecting the main conservative opposition People’s Party (PP).
But the simple majority of Socialists, anti-austerity and regional nationalist parties that united at the time against Mr Rajoy crumbled in February after Mr Sanchez refused to accept talks over Catalonia’s self-determination as part of the ongoing dialogue with separatists.
To win an outright parliamentary majority on 28 April, any one party needs to secure at least 176 seats.
The projection – taken as an average of dozens opinion polls - shows the socialists and the far-left platform Podemos (on 13.8 per cent, or 36 seats) together would reach 149 seats – 27 seats short and leaving them to rely on the support of smaller, regional parties to form a majority government.
The PP saw its support drop to 20 per cent, or roughly 75 seats, in the polls.
And with the centre-right Ciudadanos also falling to 16.3 per cent, or 58 seats, even a coalition of the three right-wing parties would fall short of a majority in parliament.
Additional reporting by agencies
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