The first vote on the assisted dying bill is not only hugely consequential, it’s also hugely unpredictable and even as the vote draws near it still feels like it could go either way.

MPs will debate the bill, brought forward by Labour MP Kim Leadbeater, in parliament today before they get a free vote on the legislation.

There are a few reasons why the potential outcome of the vote is difficult to predict. Firstly, the last Commons vote on this issue was back in 2015. It was also a Private Members’ Bill and a free vote, that was defeated by 331 to 119 – 199 MPs didn’t vote and one abstained.

That may seem like a useful starting point to predict future results but there has been an unprecedented turnover of MPs since then.

Politics latest: Starmer blames Tories for ‘open borders’

It was less than a decade ago but over two-thirds of those MPs from 2015 are no longer in parliament. This means there’s no voting record that can help us out this time round.

Secondly, it’s a free vote so we can’t, as we usually would, look to the political parties to work out the numbers.

Every single one of the 650 MPs must make up their minds for themselves and they have all taken a slightly different approach to the process.

Some came out straight away and declared their position publicly. Some took their time and have only decided in the last few days, putting out statements on social media platforms.

There are also those who prefer to keep it to themselves, and some who are genuinely still undecided and will be until they walk through the voting lobbies.

So, to get a sense of what could happen, at Sky News we have been monitoring declarations as well as reaching out to every MP personally.

This has given us, on the eve of the second reading, an informative but still incomplete picture.

So far we have confirmed that 181 MPs will vote for the bill, while 148 say they will vote against, and 300 are either undecided or haven’t revealed their decision.

There are also 20 MPs that won’t vote – the SNP because the changes won’t apply in Scotland, Sinn Fein who don’t sit in Westminster, and the Speaker and Deputy Speakers.

Of those who will vote but whose position is still unknown, about two-thirds are Labour MPs – a big chunk of those are brand new.

This is the deciding cohort, who just a few months into their roles will make a life-or-death decision that will influence generations to come – no pressure.

Ms Leadbeater has said she hopes parliament will “show itself at its best” by voting in favour of the bill.

In a statement on Thursday night, she said: “I hope this parliament will also be remembered for this major social reform that gives people autonomy over the end of their lives and puts right an injustice that has been left on the statute books for far too long.

“People will be looking in on parliament as it debates this important change to the law – a change that, when we most need it, could bring comfort to any one of us or to somebody we love.”

Read more on assisted dying:
Analysis: Bill could be wrecked by dirty tricks
Five stories that bring the assisted dying debate home
David Cameron comes out in support of bill
How MPs are making up their minds
What does the bill propose?

What could make the difference?

Most MPs tell us they have been poring over the legislation line-by-line and listening intently to their constituents.

But beyond that, there are external factors that will no doubt have influenced their thinking.

Public opinion will be high on the list, with the latest YouGov poll – one of many – showing an overwhelming majority (73%) of the public are in favour of a change in the law.

The other will be how Cabinet ministers vote, with many high profile and respected names, Ed Miliband and Hilary Benn among them, coming out in favour.

More controversial though are those who oppose the bill.

In particular, the Health Secretary Wes Streeting and the Justice Secretary Shabana Mahmood have made the news with their views.

They will both have to take a leading role in implementing the legislation if it passes.

Mr Streeting shocked Westminster when he revealed he had changed his mind after voting for the 2015 version.

He also ruffled feathers among colleagues when he appeared to breach the etiquette around free votes, by repeatedly raising concerns around extra pressures on the NHS and making the case for improving palliative care instead.

Mr Streeting’s position and approach have made the bill’s supporters nervous that new MPs will fall in behind him.

In contrast, other big beasts – the prime minister, the chancellor and the foreign secretary – remain silent on which way they will go, aware that their opinions could sway the result.

As it stands, after all the number crunching, it looks likely that this landmark legislation will pass the second reading.

But with so many unknowns, both sides will feel that even at this late stage, it’s still impossible to call.