MADRID (Reuters) - A repeat election in Spain would bolster the two traditional mainstream parties but bring no clear majority, an opinion poll indicated on Monday, with only two weeks left for Socialist acting premier Pedro Sanchez to show he can form a government.
Sanchez, who won an election in April but fell far short of a majority, has until Sept. 23 to be confirmed by parliament - but his talks with the far-left Unidas Podemos have so far not produced an agreement to form a government.
A GAD3 poll for the ABC newspaper found the Socialists would get 137 seats in another election, up 14 from April but still far from the 176 needed for an absolute majority.
The conservative People's Party (PP), which suffered a severe defeat in April, would be the other winner, gaining 16 seats to 82 - the only other party to benefit from a return to the ballot box.
The upstart populist parties that over the past five years have broken the big two parties' decades of dominance would lose out, the survey suggested.
Vox, the first far-right party to gain a sizable presence in parliament since Spain's return to democracy, would lose 10 of its 24 seats, while Podemos would lose seven of its 42 seats and the centre-right Ciudadanos would lose 12 of its current 57.
According to the poll, Sanchez would no longer need the support of the Catalan separatist parties to rule, but would still require the votes of the Basque regional party PNV and those of Podemos, which last week dismissed his latest offer.
GAD3 polled 1,000 people between Sept. 2 and 6.
(Reporting by Jose Elías Rodríguez and Ingrid Melander; Editing by Kevin Liffey)