Sir Keir Starmer will launch his Labourleadership campaign with a pledge to win back former supporters who switched to the Tories at the general election.
At a launch event in Manchester on Saturday, the shadow Brexit secretary will say the party must retain its radicalism while making it relevant to people's everyday lives.
He will promise to pursue Boris Johnson relentlessly in Parliament if he succeeds in becoming leader when the result is announced in April.
Sir Keir has emerged as the clear favourite among LabourMPs and was the first of the six contenders to secure the 22 nominations required to progress to the next stage of the contest.
He has, however, been keen to emphasise his radical credentials in a pitch to grassroots party members who are thought to be more left-wing and whose votes will ultimately determine the outcome.
Speaking ahead of the launch, he said: "The challenge for Labourtoday is to defend our values, retain our radicalism, and to make that relevant to people's everyday lives.
"People desperately need and want us to win. We have to deliver a programme that will tackle low pay and insecure work, rebuild our public services, empower communities and tackle the climate emergency.
"We have to rebuild people's trust in Labouras a force for good and real change."
He said the party needed to set its sights on winning back the support of voters across England, Scotland and Wales who turned away from it at the election in December.
"Boris Johnson said that many voters in the North had lent their votes to the Conservatives. We must set ourselves the task of winning those votes back," he said.
"If elected leader, I will pursue Boris Johnson relentlessly in Parliament. We will connect our opposition in Parliament to Labour's mass movement and to the wider forces in our country who do not want five more years of Tory misrule.
"And we will bind that together with an electoral strategy that focuses tirelessly on getting Labour back into government."