Keir Starmer has sent an open letter to the country urging them to reject Tory sleaze at Thursday's elections.
The Labour leader will call on the Government to "overhaul" the system which allowed taxpayer's cash to be wasted on contracts to cronies.
Boris Johnson has been mired in a series of rows in recent days over the financing of the lavish refurb of his Downing Street flat, as well as allegations a Tory donor was asked to pay for his son Wilfred's nanny.
The Government has also faced difficult questions over multi-million pound Covid contracts going to businesses with Tory links.
Three polls in a row have now shown the Tory lead has been dented over "cash for curtains" row - suggesting claims by the party that the row was not "cutting through" may have been wide of the mark.
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But Labour still faces a challenge hanging onto Hartlepool in a by-election on Thursday and to win the West Midlands mayoralty at the heart of the Red Wall, as well as seats in the Scottish elections.
But Sir Keir writes: “I get angry when I hear how the friend and neighbour of a Tory minister gets £30million of taxpayers’ money, while towns and communities across the country see their local services cut.
"Or when I see the British people’s money wasted on government contracts that don’t deliver.
"If I were Prime Minister, I would stop the abuse of taxpayers’ money, stop the wasteful approach to outsourcing contracts and clean up our politics.
"When I was Director of Public Prosecutions, I was not afraid to prosecute MPs who had broken the rules over MPs’ expenses.
"As Prime Minister, I would not be afraid to overhaul a system that still allows power to be abused.”
One of Mr Johnson's allies said he could refuse to stand down even if he has found to have broken the ministerial code with the lavish refurb of his flat.
Foreign Office minister James Cleverly said it would not be "as straightforward" as quitting if an independent report decides he has breached the rules on ministers' expected conduct in office.
But Scots Tory leader Douglas Ross has said the PM should quit if he is found to have broken the rules.
