Labour’s smashing victory in the Rutherglen and West Hamilton by-election is just the boost Sir Keir Starmer was hoping for as he heads to his party conference, where he wants to cement his image as a prime minister in waiting. 

It is the first time Labour have taken a seat from the SNP at a Westminster by-election.

If repeated at the approaching general election, the 20% swing to Michael Shanks the new MP, would give Labour some 40 seats, returning the party to the dominance it enjoyed before the surge of Scottish nationalism in the past decade.

Even half that total would give Labour a Tartan Wall bolstering the party’s chances of forming a majority government at Westminster rather than a hung parliament.

But, for all Labour’s excitement, it is not a done deal yet. In her introduction to the handbook for the annual Labour conference, which begins in Liverpool this weekend, Labour’s deputy leader Angela Rayner says “Labour is preparing for power with a long-term plan to give Britain back its future” but she also insists “we’ll take nothing for granted”.

Her boss, Starmer, is more cautious still, telling his party: “We’re heading in the right direction. But now is the time to step up another gear.”

This leaves Labour with two tasks as they gather in Liverpool. They want to prepare for government – by outlining their plans and showing the public that they are a trustworthy and competent team in the centre ground of British politics.

But they have got to win the next election first, and every word spoken on the banks of the Mersey will be scrutinized as to whether it is likely to attract or repel voters.

Many Tory MPs, business representatives and lobbyists did not bother to go to the Conservative conference last week in Manchester, sensing the end of an era. There will be many more sponsored stalls, receptions and fringe meetings in Liverpool because independent interests are anticipating a change of government.

Unless earth-shaking events elsewhere take attention away from the conference, Starmer’s leader’s speech at 2pm on Tuesday afternoon will be the most closely watched hour of his life. He is being auditioned as the likely next prime minister.

Starmer’s popularity lacks behind that of the party

Every focus group, vox pop and survey shows that people have not yet fully warmed to him. What people think of the party leader is a strong indicator of how people will actually vote.

Starmer’s popularity lags behind Labour’s.

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In the post-Tory conference YouGov poll for The Times, Labour is well ahead at 45% voting intention but Starmer rates only 34% as “best prime minister”. Rishi Sunak on 25% is about the same as his party’s 25%.

Word clouds showing what descriptions ordinary people attach to Sunak feature “rich”, “himself” and “money” prominently.

Starmer gets “boring”, “dull”, “untrustworthy” and “weak”. The two leaders have noticed these digs and routinely punch each other’s bruises in their exchanges.

In Liverpool, Starmer will need to be tougher on his opponent than mocking him for being super-wealthy and out of touch.

In his leader’s speech last week, Sunak tried to launch himself as the “change” candidate. He disassociated himself from the last five Conservative prime ministers and did a U-turn on the full HS2 rail programme which he, David Cameron and Boris Johnson all previously backed.

Yet until now, the public has been more receptive to charges of “flip-flopping” repeatedly levelled by Tory campaigners against Starmer.

This was at the focus of Sunak’s personal attack on him last week.

“The worst thing about Sir Keir is that he just says whatever he thinks will benefit him the most”, the prime minister told his audience in Manchester. “It doesn’t matter whether he can deliver it, doesn’t matter if it’s true, it doesn’t matter if he said the opposite just a few weeks or months ago.”

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Starmer’s service as a loyal member of Jeremy Corbyn’s shadow cabinet and his shifting policy answers to the shock of Brexit are key exhibits for the prosecution.

Starmer has dealt with the first by expelling Corbyn from Labour and abandoning the Corbynite policy pledges he made to get elected.

Starmer has dodged an obvious trap, but could face problems over Europe

Europe could yet cause problems for him in Liverpool.

Labour has only grudgingly accepted the strict limits he has placed on future ties with the EU and the leadership is fighting off calls from the grassroots for a debate on EU policy at the conference.

Labour has already endorsed Sunak’s proposal to phase out tobacco sales this century, and can easily navigate his second conference idea of reforming A-levels over the next decade.

By accepting Sunak’s abandonment of HS2 because the Tory “wrecking ball” has already done its work, Starmer has dodged the most obvious trap set for him.

The Conservatives will not be able to challenge him on how HS2 will be paid for as well as the “Network North” road and rail schemes to which Sunak says he will redirect £36bn of savings.

Shadow chancellor Rachel Reeves will keep her iron grip on spending but some in Liverpool will be disappointed by Starmer, yet again falling in line with Tory plans.

Starmer’s speech will have to go beyond ‘if not them, why us?’

To the outrage of Conservative grandees, Labour has one significant advantage this year.

The traditional annual order for conferences is Liberal Democrats first, then Labour, then Conservatives. This autumn, more by accident than design, Labour is going last.

Starmer and his team will have the last word in what is generally expected to be the last conference campaigning season before the election.

Last week, Sunak avoided talking about housing and the cost of living crisis and failed to engage with any detail with the rail and doctors strikes or how he plans to deliver net zero, “stop the boats” or engineer economic growth.

Labour have the opportunity to deal with all these topics, which top most peoples’ worry lists.

Each day the main conference debates have been termed Mission Plenaries, on the five “missions” which Starmer set at the beginning of the year: sustained growth, safe streets, the NHS, social mobility and green energy.

But Starmer’s speech will need to go beyond “if not them, why us?”

He also has to ridicule Sunak’s bid to escape the Tory record by posing as the true change candidate.

A senior Labour campaigner likened Sunak’s shift to a snake shedding its skin, pointing out that Theresa May, Boris Johnson and Liz Truss all tried the same trick of “vote for us to keep us in power so we can change everything”.

The former director of Public Prosecutions cannot afford to be “boring”. To seize the moment, he will need to summon more Neil Kinnock-style fury or Tony Blair-style scorn than he has managed so far.

Whether Starmer raises the roof or not, Labour delegates will sober up within 24-hours of leaving Liverpool. Next Thursday, the party faces daunting tests in two by-elections in England in constituencies held by the Conservatives.

In Nadine Dorries’ old seat of Mid Bedfordshire a 19% swing from the Conservatives would do it – the party took Selby & Ainsty this summer with a swing of 23%. But the main opposition parties are fighting each other and could end up splitting the anti-Tory vote.

The other contest in Tamworth, caused by the resignation of Tory whip Chris Pincher, demonstrates the changes in the political landscape that England has undergone since the last time Labour was seriously threatening an incumbent Conservative government – and the difficulty of the challenge facing Starmer and his party.

In 1996 for Sky News I covered another by-election in Tamworth – or South East Staffordshire, as pretty much the same constituency was then called.

The new Labour leader Tony Blair was riding high and Labour captured the seat from the Conservatives with a 22% swing.

The new MP Brian Jenkins held Tamworth throughout the New Labour years until 2010. Nobody is talking up Labour’s chances of victory in the by-election in Tamworth this time – even though it would take an almost identical swing this time as back then.

Tamworth and surrounding areas voted heavily for Brexit and that changed a lot. In the neighbouring Black Country, Labour now has only three of the 13 seats it held in Blair’s heyday.

Never mind the party conference hot air, Labour victories in either or both of these English by-elections would really show that Sir Keir Starmer has shifted Labour into top gear towards general election victory.