WARSAW (AFP) - Poland's President Andrzej Duda appeared to have lost his grip on victory in an upcoming election, fresh opinion polls showed, threatening a political upset for the allied right-wing government.

While April opinion polls handed Duda over 50 per cent voter support for a comfortable first-round win, he scored between 35-39 per cent support in two separate surveys published this week.

The slide came days after the main liberal Civic Platform (PO) opposition party switched its presidential candidate, tapping Warsaw Mayor Rafal Trzaskowski.

He immediately shot to second spot in the polls, setting him up for a tight round two duel with Duda, who is backed by the governing right-wing Law and Justice (PiS) party.

Originally scheduled for May 10, the election was postponed at the last minute when government and opposition parties failed to agree how to proceed amid the coronavirus pandemic.

Parliament was still struggling this week to overcome the stalemate but senior PiS and opposition officials said in separate statements that the ballot could go ahead on June 28, before Duda's five-year term ends on Aug 6.

Support for Duda sank to 35 per cent according to a May 18-19 survey by the independent PBS pollsters published on Saturday (May 23) in the liberal Gazeta Wyborcza broadsheet daily.

A Kantar poll conducted on the same days and published on Thursday gave him 39 per cent backing.

Trzaskowski scored 21 and 18 per cent support respectively in the two surveys, followed by independent centrist Szymon Holownia, who enjoyed 19 and 15 per cent backing, in a field of six main candidates.

Opposition parties had rejected PiS government plans for a May 10 universal postal ballot amid the pandemic, insisting the constitution requires voting rules to be changed at least six months before election day.

They also insist that the way powerful PiS party leader Jaroslaw Kaczynski postponed the May 10 date, via a political declaration, was illegal.

Constitutional experts say a government can only postpone an election by declaring a state of emergency or disaster, something the PiS administration has refused to do despite the unprecedented social and economic impact of the pandemic.

The election crisis comes within the broader context of long-standing EU concerns about democratic standards in Poland.

The European Commission has launched four infringement procedures against PiS-authored judicial reforms, which it says test democracy and the rule of law by undermining judicial independence.