Jeremy Corbyn will travel to Home Secretary Amber Rudd's ultra-marginal constituency later to insist the Tories have no mandate to continue in power and that Labour is a "government in waiting".
In an election-style rally in Hastings expected to attract 1,000-3,000 people, the Labour leader will highlight the electoral vulnerability of Ms Rudd, touted as a potential successor to embattled Theresa May.
In the first day of national campaigning for Labour since the election, deputy leader Tom Watson will also travel to the seat of Boris Johnson, another potential leadership candidate, in Uxbridge and South Ruislip.
Several other shadow cabinet members will travel to Tory-held marginals around the country as Labour steps up its campaign to force out the Government.
Mr Corbyn, who is later expected to address a "Tories out" anti-austerity demonstration in central London, will say in Hastings: "I am proud to be here today in Hastings where you came within a whisker, 346 votes, of painting the town red and electing Peter Chowney as a Labour MP.
"Labour gained seats across the country at the last general election, in every region and nation of Britain.
"Labour is no longer just the Official Opposition, we are a government in waiting.
"It is not good enough for politicians – including your MP, the Home Secretary – to praise the police, firefighters and NHS staff when they respond to horrific incidents like the recent terrorist attacks or the Grenfell Tower fire, and then vote to make them worse off again year after year.
"This is what happened in parliament this week. Nurses in our hospitals have had a real terms pay cut of 14% under this government and some are even having to use food banks.
"Yet the Government refused to back our calls to lift the public sector pay cap and recruit more police and firefighters.
"You can't keep our communities safe on the cheap. When you cut over 20,000 police officers, over 11,000 firefighters, and leave our hospitals in record deficit having to cut back on services.
"We have to change our economic system so that it works for the many, not just a few."
The rally is expected to be the first in a summer in which Mr Corbyn will travel the country to speak to voters in Tory-held marginals, with the Prime Minister's authority diminished and the minority Government relying on the Democratic Unionist Party (DUP) for support to stay in power.