Keir Starmer has vowed not to let Boris Johnson “off the hook” as he begins taking the fight direct to the Tories.
He said that it was not enough for Labour to be united – it also had to be the “strongest possible” opposition to the Prime Minister.
The Labour leadership frontrunner signalled his change in tactics with just over a month of the contest left to run.
Party insiders have been despairing at the length of the three-month race which they believe has let the Government to go unchallenged.
Mr Starmer’s campaign will now turn its sights on the Tories in a bid to try to fill the void until the race is over.
The Tories have overseen floods, an NHS winter crisis and the start of trade talks with the European Union since it began.
A new YouGov poll claimed that he could easily win the leadership contest in the first round, on 53% of the vote.
Left-winger Rebecca Long Bailey trails the shadow Brexit secretary on 31% with Wigan MP Lisa Nandy behind on 16%.
The shadow Brexit secretary’s campaign received a further boost when he won the backing yesterday of London mayor Sadiq Khan .
Labour’s most powerful elected politician said he was the “best person to unite” the party and take it into government.
If he wins, Mr Starmer plans to set up an ‘attack and rebuttal’ unit within Labour Party HQ to expose the Tories’ track record.
His team is understood to believe that the party needs to professionalise after its election drubbing.
And he will appoint a shadow Cabinet with “strong voices” from across the party that he will send out onto the airwaves.
His pledge for party unity, however, is likely to face an immediate challenge as he decides who to pick for his top team.
Labour insiders believe he faces a challenge ‘reining in’ general secretary Jennie Formby and Mr Corbyn’s ex-chief of staff Karie Murphy.
A bullish Mr Starmer told the Mirror: “In government, Labour can change lives. In opposition, we can’t. That is why the election result was so devastating.
“We must be the strongest possible opposition to Boris Johnson and the Tories. We must be relentless in holding Johnson to the promises he made at the election. We cannot let him off the hook.”
More than 500,000 Labour members and registered supporters across the country started receiving their ballot papers this week.