Jaroslaw Kaczynski's declaration opened the Law and Justice party's campaigning for Oct. 21 local elections in which it hopes to strengthen its grip on power.
"The Poles want to be in the European Union because that is the shortest way to bring earnings to the European level and to raise the living standards," Kaczynski said.
Law and Justice won leadership of the Polish government in 2015 on promises on improving life for ordinary Poles and fighting crime and corruption.
Since then, the Polish government has reorganized the judiciary and criticized EU rules it says are too restrictive, raising questions about the country's attachment to the 28-nation bloc.
The European Court of Justice is reviewing changes in the composition and operation of Poland's Supreme Court. Critics said the actions put Law and Justice in position to name loyalists as justices, which in turn would give the party the ability to control elections that were vetted by court.
Former Polish President Lech Walesa, a Nobel Peace Prize recipient for his work promoting democracy, told a meeting of government opponents Sunday that Law and Justice's rule was a "misfortune" that is harming Poland's interests and needs to end.
Poles should closely watch their elections and vote to "win back a free and democratic Poland" where the judicial, legislative and executive branches of government are separate, Walesa said.
The government denies its moves were anti-democratic and has indicated it might disregard the EU court's ruling.
Kaczynski, a Catholic who opposes abortion and same-sex marriage, said Sunday that the EU membership Poland has held since 2004 benefits the country economically.
The party's goal is to have Poles say "in 15-20 years' time that things in Poland are the same as they are west of our borders, in every respect," Kaczynski said.
Law and Justice remains Poland's most popular political force thanks to increased social spending and steps it argues are eliminating poverty, the misuse of state money and corruption in the justice system.
Amid chants of "Jaroslaw, Jaroslaw" and flowing white-and-red national flags, Kaczynski said the public approval should last through Poland's parliamentary election next year and the 2020 presidential election.
Copyright 2018 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.