The Labour leader, who has been in office for almost a year, has been buffeted by criticism from his own side in recent weeks.
In a speech on Thursday widely seen as an attempt to reset the agenda, Starmer will accuse Boris Johnson of laying down “a roadmap to yesterday” with his economic plans.
But ahead of the speech, one veteran Labour MP accused Starmer of too narrow a focus on the “owners of capital”, while another said he supported the message but that the party must “go further” and give “power to people”.
Starmer will say: “In a few weeks’ time, we will have a Budget that will be a fork in the road.
“We can go back to the same insecure and unequal economy that has been so cruelly exposed by the virus, or we can seize this moment and go forward to a future that is going to look utterly unlike the past.
“That choice will define the Budget and it will define the next election.”
Earlier this month left-wing MPs, including former shadow chancellor John McDonnell, demanded an emergency party conference be held amid “anger and disillusionment” at the direction the party was taking.
On Tuesday, party activist Tom Kibasi, who helped Starmer in his leadership campaign, accused the Labour leader of launching an “completely unnecessary war with the party’s left”.
In an article in The Guardian, Kibasi said the Labour leader “would not leave a trace of a meaningful political project in his wake” if he resigned tomorrow.
Starmer was also forced to reject claims that Labour was trying to imitate “the nativist right” by wrapping itself in the Union Jack.
Jon Trickett, who served in several shadow cabinet roles under Jeremy Corbyn, told HuffPost UK: “The kind of vision I would like for Labour is one which offers a partnership between government and the wider society, not focusing simply on the owners of capital.
“I am thinking of a partnership with the NHS, the educators, voluntary sector, and the millions of frontline workers who have seen us through the pandemic. Such an alliance would build the truly fair and just society which is bursting to be born as we move beyond Covid.”
Labour MP Clive Lewis, who has accused Starmer of “absorbing the language and symbols of the Tory party”, said it was “great” the party leader was “laying out his diagnosis”, which he supported, but he would “go further”.
“We’ve been letting our infrastructure weaken for more than the last decade. Just ask the voters in many of those ‘foundation seats’ if they think it’s only been the last 10 years things haven’t been working out for their communities,” he told HuffPost UK.
“I think you’ll find our country’s problems go back further than that.
“I think for the future we need not just a partnership between government and business, as important as that is – we need to create ways for people all over the country to be involved in both economic and political ‘levelling up’.
“Keir is right that government and active state play a vital role but that doesn’t mean more Westminster and Whitehall, it means more power to people in their towns and workplaces.”
But amid the criticism, one key aide to Starmer dismissed left-wing attacks on his leadership as not based in “the real world”.
Carolyn Harris, the Labour leader’s parliamentary private secretary (PPS), told HuffPost UK’s Commons People podcast recently that left-wing critics of Starmer are “making themselves look silly”.
“All this nonsense about ‘so and so’ and ‘so and so’ is going to challenge him for the leadership – bring it on, because it’s nonsense,” she said.
In the speech, Starmer will demand there be “no return to business as usual”, but will also seek to address any accusations Labour will abandon what he calls “financial responsibility”.
“I know the value of people’s hard-earned money – I take that incredibly seriously – and I know that people rightly expect the government to look after it too,” he will say.
“To invest wisely and not to spend money we can’t afford. Those are my guiding principles. But I think that Covid has shifted the axis on economic policy: both what is necessary and what is possible have changed.
“I believe people are now looking for more from their government – like they were after the Second World War.
“They’re looking for government to help them through difficult times, to provide security and to build a better future for them and their families.
“Despite the scale of the moment, all we can expect from this government is more of the same. A roadmap to yesterday.”