Sir Keir Starmer says he will take "full responsibility" for Labour's results at Thursday's elections.
The leader says there is a "mountain to climb" after the party's poor general election results back in 2019 and the next poll is the "first step".
He pledged to "clean up" politics after the "return of Tory sleaze" - pointing to recent allegations around cronyism and lobbying in Westminster.
Senior minister Liz Truss said voters were focused on other issues.
"What people care about is how we're recovering the economy from Covid, and jobs and growth across the United Kingdom," she added.
Every voter in England, Scotland and Wales will have the chance to take part in at least one election on Thursday.
As well as polls for Holyrood and the Senedd, English council seats will be up for grabs, along with 13 directly elected mayors, the London Assembly and 39 Police and Crime Commissioner posts.
There will also be a by-election for a new MP in the seat of Hartlepool.
Sir Keir took over Labour 13 months ago and Thursday's visit to the polls will be the first electoral test of his leadership.
Speaking in a series of BBC interviews, he insisted "no-one thought it possible to rebuild in a year or so" after the party's worst performance in a general election since 1935.
He added: "I said on the day that I was elected that that was a mountain to climb. It is, we're climbing it and I've got a burning desire to build a better future for our country, and Thursday is a first step towards that better future."
But asked if he accepted this week's elections would reflect on him, Sir Keir said: "Yes, and I take full responsibility for the results, just as I take full responsibility for everything that happens in the Labour Party under my leadership."
Sir Keir outlined a number of Labour's policies, including:
- A £30bn investment into "new jobs in the industries of the future"
- A "guarantee" to get young people into work, training or education after six months of unemployment
- A "proper" pay rise for NHS workers
Sir Keir was also keen to echo the approach of new US President Joe Biden, saying "short termism has bedevilled our economy" and "preventative" measures - such as addressing the roots of crime through education and housing - would be more cost effective in the long term.
"The inequality that's built into our economic model is morally unjust, but it's economically stupid," he added.
But asked how such plans would be paid for, the Labour leader said we would have to wait to see "a fully costed manifesto at the next election".
Meanwhile, the government has confirmed it is preparing a white paper - a document laying out a policy - on "levelling up", which will be published later this year.
A Downing Street spokesman said the plans would be led by the prime minister and focus on challenges such as improving living standards, growing the private sector, and increasing and spreading opportunity.
A new unit will be set up jointly between No 10 and the Cabinet Office to work on the policy, and Tory MP Neil O'Brien has been appointed as the PM's "levelling up" adviser.
Labour leader Sir Keir also responded to criticism of pictures of him in John Lewis last week, where Tories accused him of "playing politics" over the row about Boris Johnson's Downing Street flat renovations.
Asked if the move was misjudged, Sir Keir said: "I don't turn my nose up at John Lewis. I am proud to support John Lewis. What I was pointing to there was the evidence we have seen in the last few weeks in particular of the return of Tory sleaze."
He attacked the concept that allegations over Mr Johnson's conduct in office were "priced in", saying: "Being the prime minister of the United Kingdom is an incredible honour and privilege and duty, and it shouldn't be 'priced in' that the prime minister is not being straight [with us].
"I think there is a very strong feeling across the country, particularly on the back of the pandemic, that this idea that some of the top of government seem to have that the rules don't really apply to them... is completely wrong."
But also speaking to the BBC, International Trade Secretary Ms Truss said the accusations were not the focus on the doorstep.
She added: "I've been out campaigning in the local elections and what I'm hearing from voters is people are delighted by the progress of the Covid vaccination programme - we've just seen 50 million vaccinations done across the UK.
"What people care about is how we're recovering the economy from Covid, and jobs and growth across the United Kingdom. We've secured 6,000 jobs from our new enhanced trade partnership with India.
"Those are the issues that will motivate voters when they go to the polling stations this Thursday."