The Labour government was already limping along far short of the votes needed to keep its parliamentary majority long before Jeremy Corbyn and Zarah Sultana announced the formation of Your Party. This month's polls show its vote down by more than a third from what won it the general election just a year ago.
Defections from Labour started last autumn when Rachel Reeves made clear that the government's priority was fiscal caution. By October its poll support averaged 29 percent, insufficient to maintain its parliamentary majority. By this February Labour had fallen behind Reform UK in the polls and since May it has lagged beyond Reform's predicted number of MPs. The current Electoral Calculus prediction is a Reform majority of 186 seats over Labour.
Your Party must compete with three other parties for the vote of Labour defectors. Most Labour voters tend to be centre-left or soft-left rather than hard-left like Your Party's platform. YouGov's July survey reports twelve percent of voters who helped Labour to its massive general election victory now support the Liberals, ten percent Reform and three percent he Tories. Against this, eleven percent have defected to the Greens.
There remains an untapped group of left-wing Labour supporters who are open to defecting to Your Party. Estimates run as high as ten percent of Labour's current voters. This suggests Your Party might reduce Labour's vote by 2.25 percent of the national vote.
Adding Your Party's straw of votes to the pile of Labour defectors could cost the government 27 seats, according to Electoral Calculus. This would have little effect on Labour's political standing. If a general election had been held the day before Your Party was launched, Labour was already threatened with having fewer MPs than at any election since 1931.
The biggest beneficiary from the launch of a new left-wing party is Reform UK. It would add 25 seats to its notional majority while the Conservatives would gain only one seat if Labour fell to a fifth of the national vote. Whether the Greens would gain or lose a seat or two would depend on whether Your Party, which supports radical green policies, would stand down in their strongest constituencies.
A forecast of Your Party's national vote assumes the party would nominate candidates nationwide. There is no need to do so, for its candidates would lose their deposits and secure derisory votes in hundreds of seats where it finishes third or worse behind Conservatives, Liberal Democrats or other parties.
At the constituency level Your Party's two leaders are well placed to remain MPs. Jeremy Corbyn's personal appeal has enabled him to win re-election as an independent. Many Your Party candidates will be running on a platform that is even more left-wing than the 1983 Labour manifesto of Michael Foot, described as the longest suicide note in history.
Zarah Sultana's plan to abandon her Coventry constituency to fight Justice Minister Shabana Mahmood in Birmingham Ladywood is shrewd. At the last election an independent candidate standing on a pro-Gaza platform nearly halved the Labour minister's vote. As that candidate is now under indictment, Sultana has a good chance to fill that vacancy.
There are up to a dozen or more constituencies with Muslim electors numerous enough so that a Your Party candidate might take a seat from a sitting Labour MP by being more pro-Palestine than the official party line. That is why Keir Starmer did another U-turn this week, announcing the Labour government would recognise a Palestinian state.
Prof Richard Rose, University of Strathclyde, is currently writing "The Breakdown of the British Party System: a Guide to the 2029 Election".