LISBON (Reuters) - Portugal's main opposition party, the centre-right Social Democrats (PSD) secured a marginal lead over the ruling Socialist party after surging in the polls just days before a snap election, a survey published on Tuesday showed.

       The opinion poll carried out by Aximage showed a huge jump in support for the Social Democrats (PSD), led by moderate economist Rui Rio, to 34.4% from 28.5% a week ago.

       Prime Minister Antonio Costa's centre-left Socialists on the other hand suffered a big decline, with 33.8% share of the vote compared to 38.5% earlier, confirming loss of ground shown also in other polls.

       The PSD has been gaining ground since emerging from a long period of internal turmoil in November, when it reelected Rio as its leader, and running a strong campaign.

       Their growing chances, and the Socialists' wear and tear from managing the COVID-19 pandemic, may have swayed many undecided voters lately, analysts say, although the election remains wide open.

       Still, both parties polled well shy of a working majority, which means they will have to negotiate post-election alliances.

       Costa has said he would leave his party leadership if he loses the election.

       In October, Costa's two former allies - the Communists and Left Bloc - sided with right-wing parties to reject the minority government's budget bill, triggering the snap election.

       The Left Bloc and the Communists polled at 6.6% and 4.5% respectively in the Aximage survey.

       The far-right party Chega would become the third-largest force in parliament, polling at 8%.

       The poll put the share of voting intentions for the Liberal Initiative party at 2.8%, while the right-wing CDS-PP could win 1.6%.

       Aximage's poll gave the People-Animals-Nature (PAN) party 3.3% support and the eco-Socialist Livre 1.4%.

       Political analysts say the election alone might not solve the potential impasse as no party or workable alliance is likely to achieve a stable majority, potentially undermining the country's ability to spur growth using European pandemic recovery funds.

       The Aximage poll, which was conducted for newspapers Jornal de Noticias, Diario de Noticias and TSF radio, surveyed 965 people on Jan. 16-21.