An opinion poll published on Friday, the last day of the campaign, showed PO, broadly pro-business and supportive of closer ties with the European Union, comfortably ahead of its main rival, the nationalist Law and Justice party (PiS).

The survey, conducted by the Estymator pollster for the Polish edition of Newsweek magazine, showed PO winning 35 percent against 27 percent for PiS, a Euroskeptical party that favors more state involvement in the economy.

“At stake in this election are security and the stable development of our country. In my view, only PO guarantees that,” Tusk told reporters at Warsaw’s new National Stadium, one of the venues for the Euro 2012 soccer tournament.

The stadium is a symbol of the building boom — much of it financed by EU funds — that Tusk has overseen since 2007.

Polls this week have shown PO widening its lead over PiS but Tusk knows he cannot be complacent. In 2005, polls also put his party ahead but it finally lost to Jaroslaw Kaczynski’s PiS.

“We have to fight to the very end, I’m clear that nothing is decided yet. I am glad that PO got a fresh impetus in the final countdown... but we should prejudge nothing,” said Tusk at the stadium, where he was surrounded by school children.

Many analysts say support for Kaczynski has ebbed after he made remarks about Germany and Chancellor Angela Merkel in a new book that his rivals said were offensive.

Kaczynski repeated in his book his long-held view that Germany wants to subdue Poland and also said “it was not a matter of pure coincidence that Merkel was elected chancellor.” He has declined to clarify what he meant by the remark.

The winner of Sunday’s poll will almost certainly need to build a coalition with one or more of the smaller parties also expected to clear the 5 percent threshold to enter parliament.

These include the Peasants’ Party (PSL) — PO’s current coalition partner — the post-communist Democratic Left Alliance (SLD) and a new libertarian party founded by maverick ex-PO lawmaker Janusz Palikot that backs gay rights and abortion.

Tusk wants to continue his alliance with PSL and has spoken strongly against teaming with Palikot.

PO sources say PO and PSL will try to cherry-pick some leftist MPs if together they are just short of a parliamentary majority to avoid a coalition with the whole of SLD.

Tusk has campaigned on Poland’s strong economic growth, which is expected to hit 4 percent this year.

He has also tried to scare voters with memories of the last Kaczynski-led government in 2006-7, which was marred by clumsy efforts to purge or prosecute communist-era officials and by rows with Poland’s powerful neighbors Russia and Germany.

Tusk vowed to continue a steady rapprochement with Russia despite disagreements on issues from missile defense to gas pipelines and over the conduct of an investigation into a plane crash that killed Poland’s president last year.

President Lech Kaczynski — Jaroslaw’s twin brother — and nearly 100 other senior officials, many from PiS, were killed in the crash near Smolensk in western Russia on April 10, 2010.

Moscow blamed the Polish pilots but Warsaw said Russian ground control also bore some blame for the tragedy. Jaroslaw Kaczynski blames both Moscow and the Tusk government.

“The Smolensk catastrophe was certainly a drama and the way it was then investigated did not make it easier for us to seek a full reset with Russia,” Tusk said.

“Poland does not plan to reset its memory, but Poland and Russia deserve friendly relations.”

Kaczynski renewed his attacks on Friday against PO plans to step up privatization and to introduce an element of competition into Poland’s ramshackle public health system.

“If Poland becomes a strong, developed country, we won’t have to privatise Polish companies, sell state firms for peanuts or privatise hospitals,” he told the Fakt tabloid.