Today’s by-election in Gorton and Denton is much more than a vote to select one of Westminster’s 650 MPs. The result will shape the strategy and pitches of parties all the way up to the next general election. It could also determine the fate of the Prime Minister.

For Labour and Reform UK, this is a crucial contest. Sir Keir Starmer needs to show that under his leadership Labour can unite the anti-Nigel Farage vote and hold onto a once rock solid constituency. If he cannot and the seat is won by Reform’s Matt Goodwin or the Greens’ Hannah Spencer then Sir Keir will be blamed for a humiliating defeat.

The soft Left of the party will say that if Labour’s National Executive Committee had allowed Andy Burnham, the Mayor of Greater Manchester, to stand the seat would still be painted red.

The election has played out against the unfolding of the Mandelson-Epstein saga, and there is still dismay in Labour ranks that Sir Keir appointed the New Labour strategist as Britain’s ambassador to the United States. A loss in a seat that Labour won in July 2024 with a majority of 13,413 will deepen the sense in that a true reboot of the party is needed, and a change of leader is a necessity. But if Labour’s Angeliki Stogia delivers a win she will buy Sir Keir time and encourage her new parliamentary comrades to hope that Zack Polanski’s Greens can be beaten back.

The stakes are also high for Reform. The party will want to repeat last year’s Runcorn by-election success, when Sarah Pochin won by six votes. A win is a win, and the addition of a new member on the Reform benches – not as a result of a Tory defection but through an election victory – will put new momentum behind a party intent on seizing power.

At present, it is suggested only a few hundred ballots could decide the result. Reform’s get out the vote operation will strive to avoid the disappointment of last year’s Caerphilly by-election. In this race for a seat in the Welsh Parliament Labour voters deserted Sir Keir’s party but they swung behind Plaid Cymru and handed the pro-independence party a majority of 3,848.

If Left-leaning voters unite to deny Reform victory and stop Mr Goodwin – one of Britain’s most unapologetic and articulate advocates of populism – becoming an MP, then lessons will be learned at the party’s HQ. Recent polling shows the vast majority of Labour voters are prepared to vote tactically; this is one of the biggest threats to Mr Farage becoming prime minister.

There will be groans on the Right at the suggestion a Left-leaning alliance of Labour, Liberal Democrat and Green MPs – perhaps in some arrangement with the nationalist parties – could wield power after the next Westminster election.

But it is clear the days when Labour could take supposedly safe seats for granted are over, and the coming years will be defined by an epic battle for your trust between rival parties with radically different visions for the future of Britain.