Wales needs a UK government on its side, Labour leader Jeremy Corbyn has said during a summer campaign tour of marginal seats.
He promised investment in jobs and industry at a rally in Bangor, after meeting community groups in Llandudno.
Labour came within 92 votes of taking the Arfon seat from Plaid Cymru and cut the Tory majority in Aberconwy to 635.
The Welsh Conservatives said his plans would lead to "bankrupt public finances".
Ahead of Saturday's visit, Mr Corbyn said the next Labour government would "transform" the economy, building on the new Development Bank of Wales.
"We will develop the jobs, skills, infrastructure and industries of the future through an investment-led approach, supported by our National Transformation Fund and a Welsh development bank, building on the Welsh Labour Government's Development Bank of Wales," Mr Corbyn said.
"People in Wales need a government in Westminster that is on their side," he added, pledging a £10 minimum wage, an end to the public sector pay cap and action on energy prices and rail fares.
Labour gained three seats from the Conservatives in Wales in June's election as Prime Minister Theresa May lost her majority at Westminster.
Conservative Guto Bebb and Plaid's Hywel Williams were both re-elected for Aberconwy and Arfon respectively but both saw majorities of almost 4,000 over second-place Labour cut.
Targeting voters in the two seats, Mr Corbyn said: "We can win here and form the next government that will work for the many not the few."
The Welsh Conservatives said Mr Corbyn's plans would lead to "bankrupt public finances" and "broken public services".
The party's leader Andrew RT Davies said: "In Wales we've endured a Labour-led government for the past 18 years with take-home pay still the lowest in the UK and communities left behind by empty rhetoric and broken promises.
"Today, hardworking people across north Wales have the opportunity to shine a spotlight on the failures of the Labour Party over the past two decades and will rightly be excused for pressing Jeremy Corbyn on the shortcomings of his colleague Carwyn Jones in Cardiff Bay."