WARSAW (Reuters) - Poland's nationalist ruling party pledged on Saturday to increase public spending by up to $10 billion a year, raising child subsidies, state pensions and transport infrastructure as part of a campaign ahead of this year's parliamentary election.

The eurosceptic Law and Justice (PiS) party won power in 2015 in part because of wide public support for its expansive welfare programs and promises of more economic equality.

With a general election due in late 2019, PiS remains the most popular party in Poland but a string of scandals has eroded its support, raising some doubts whether it can retain its parliamentary majority. PiS leader Jaroslaw Kaczynski said the party was offering voters all it could.

"A person whose pockets are empty isn't free," Kaczynski, Poland's de facto leader, told supporters at a party convention. "We are filling these pockets, within what's possible."

Kaczynski said most of the spending increases would take place in coming months and include an income tax exemption for workers younger than 26 years old.

Prime Minister Mateusz Morawiecki said the proposals would increase state spending by 30 billion-40 billion zloty ($7.8 billion-$10.5 billion) a year. Poland's 2019 budget deficit is capped at 28.5 billion zloty.

($1 = 3.8258 zlotys)

(Reporting by Justyna Pawlak; Editing by Peter Graff)