Zack Polanski is facing a fight for survival in only his second Green Party conference speech as leader amid the party's plummeting opinion poll ratings.
The self-described 'eco-populist' saw an initial surge in support for his party following his election as leader little more than a year ago.
But the 43-year-old has seen the Greens' poll rating almost halve since a peak in March, amid a series of rows and Labour's lurch to the Left under Andy Burnham.
The start of the Greens' conference in Brighton this weekend has been accompanied by rumours of a potential leadership challenge from one of Mr Polanski's deputies.
Mothin Ali, a Leeds City councillor, is reported to be 'gearing up' to run in the next Green leadership election due next year.
There has also been a dispute over whether TV channels can air Mr Polanski's keynote speech live on Friday afternoon, due to strict by-election rules.
Meanwhile, the Brighton gathering is set to be dominated by a furious row over a planned debate by Green members on a motion to declare Zionism as a form of racism.
Zack Polanski is facing a fight for survival in only his second Green Party conference speech as leader amid the party's plummeting opinion poll ratings
Caroline Lucas, the former Green leader, has branded the motion 'divisive' and warned it 'could be seen as a sign that Jewish people aren't welcome in the party'.
Speaking at her party's own conference in Liverpool this week, Labour deputy leader Lucy Powell said she is 'confident' of winning the Holborn and St Pancras contest.
She claimed many Labour voters who left the party for the Greens during Sir Keir's premiership were giving the party 'a second look' now Mr Burnham is in power.
A defeat for Mr Polanski in next Thursday's by-election would represent a huge blow to his leadership.
He is reported to have requested that the Greens’ ruling executive committee postpone the party’s conference in Brighton so they could focus on the Holborn and St Pancras contest but was refused.
Rob Ford, professor of political science at the University of Manchester, said Mr Polanski's first year as Green leader had 'brought many more voters off the fence, but most have broken against him'.
'This big negative shift has coincided with Labour installing a more popular leader and with a large positive shift in Conservative leader Kemi Badenoch's ratings,' he wrote in an article for Sky News.
'Polanski and the Greens need to decide how to respond to Labour's moves to close down space on the Left.'
He added: 'Regardless of which way they turn, the Greens may also need to ask themselves whether Polanski remains a political asset after a turbulent first year as leader, which has seen his poll ratings fade fast.
'Though Labour gains from the Greens are much more evident since Andy Burnham became PM, Green polling had started to decline even before his arrival.'
However, Rachel Millward - another Green deputy leader - claimed her party and Mr Polanski had 'set the political agenda' in the past 12 months.
She told Times Radio: 'The reality is that it's Zack Polanski who brought the nation Andy Burnham.
'And if we hadn't had the success of the Green Party in this last year since Zack's been leading, which has been unprecedented, phenomenal success...
'...If we hadn't had that much pressure on Labour, then we'd probably still be in the Starmer regime, which was pretty much ushering in a Reform government with its lurch to the Right.
'So actually, what we're seeing now is that we've set the political agenda and the Government is picking that up.'
In his conference speech on Friday, Mr Polanski is expected to call for a triple lock-style guarantee for private renters to ease young adults' housing costs.
He will set out plans for a 'three-year emergency brake', which would temporarily limit rent rises to the lowest of either Consumer Prices Index (CPI) inflation, wage growth or 2 per cent.
These are similar metrics to the ones used to uprate the state pension each year, which rises by the highest of either inflation, wage growth or 2.5 per cent.
'For too long, landlords have been able to hike prices way beyond wages,' Mr Polanski is expected to say.
He will describe '25-year-olds who can't find a job, no matter how many tens or hundreds of applications they put in, who can't imagine ever being able to afford to rent even a room of their own, never mind being able to put down roots in their own home'.
The party leader will also describe '30, 35-year-olds moving back to their parents' place with their partners in tow, sometimes their kids too', warning that for many of them 'times get tougher and those months turn to years'.
Mr Polanski is expected to say: 'Today I'm calling for a three-year emergency brake on private rents – no rent rises above CPI, wage growth or 2 per cent – whichever is lower.
'No sudden hikes, no evictions under the guise of rent rises, no more fear and uncertainty.
'That emergency brake would give renters vital breathing space. Let wages begin to catch up with the rents that have outstripped them for too long.
'And give government time to develop a national fair rents guarantee – to keep rents fair and affordable for everyone, forever.'