MADRID (Reuters) - Spain’s left-wing parties could win a majority in elections on April 28, although that is not guaranteed, while right-wing parties look set to fall short under any scenario, according to an opinion poll on Tuesday.

Spain is facing one of its most bitterly fought parliamentary elections in decades, and polls have been suggesting for weeks that the result is too close to call.

The state-run Centre for Sociological Studies (CIS) is forecasting a wide range of 157-181 seats for the ruling Socialists together with the anti-austerity Podemos and smaller left-wing allies.

Under the best-case scenario, they would together secure a majority in the 350-seat parliament.

But they might not be able to rule on their own and would have to seek allies among regional parties in Catalonia or the Basque country, a difficult undertaking that would require lengthy talks.

The election is expected to mark a watershed in Spain’s democratic history as a far-right party looks set to win seats for the first time in four decades.

The CIS poll, the last before the election, sees that party, Vox, gaining 29 to 37 seats.

The conservative People’s Party is seen getting 66-76 seats and the center-right Ciudadanos 42-51.

This means that the poll does not see the three right-wing parties winning a majority of seats, even under the most optimistic scenario.

A Ciudadanos-Socialist alliance would theoretically get a majority, but Ciudadanos leader Albert Rivera has ruled out such a pact.

The poll also indicated that one in four voters had not yet decided who to vote for, and more than eight percent gave no answer, showing that things could still change.

Spanish pollsters have traditionally struggled to predict seats for smaller or new parties such as Vox, and they disagree on what the outcome will be. tmsnrt.rs/2ENugtw

CIS polled 16,194 people from March 1-18, after Socialist Prime Minister Pedro Sanchez called the snap election.

The poll shows important changes in seats for most parties.

The Socialists would see a big jump from their current 84 seats to 123-138, while the conservative People’s Party could lose about half its seats.

Podemos and regional offshoots would see their seats drop from 71 to a maximum of 43.