CNN  — 

Chalk up a big victory for Boris Johnson in Brussels: the UK Prime Minister got the European Union to renegotiate the withdrawal agreement that the EU said it would never renegotiate.

But, as his predecessor Theresa May proved, getting a deal between the UK and the EU isn’t the hard part – getting the British Parliament to back it is. So can Johnson pull another rabbit out of his hat?

Let’s look at the numbers

There are 650 members of Parliament, but seven of them belong to the Irish nationalist Sinn Fein party. On principle, they never take their seats.

The Speaker of the Commons, John Bercow (you’ve heard him shouting “Order, order!”), and his three deputies also don’t vote.

That leaves 639 voting MPs, which means Johnson needs 320 – a simple majority – to get his Brexit deal through.

Or to put it another way, if 320 lawmakers vote against it, it’s dead.

The opposition Labour Party says it won’t back it. There are 244 Labour MPs, but a handful of them voted for Theresa May’s deal and might vote for Boris Johnson’s. At least three Labour MPs – Ronnie Campbell, Jim Fitzpatrick and John Mann – have already pledged their support for it.

So let’s say around 240 Labour MPs vote against – although Johnson will actually be hoping to persuade more to swing to his side.

Arlene Foster, leader of the Democratic Unionist Party, has said "no" to Johnson's deal.

The Democratic Unionist Party, which props up the Conservative government, says they won’t vote for the deal. There are only 10 of them, led by Arlene Foster, but they’re a disciplined bunch so if they say they won’t vote for it, they probably won’t.

That makes 250 against.

The Scottish Nationalist Party, Plaid Cymru, Independent Group for Change and the one Green MP are all likely to vote against the Johnson deal – another 45 against, for a total of about 295.

Then there are the Liberal Democrats, who are riding high on being the main national party that unequivocally opposes Brexit. There are 19 of them – although one rebellious member, Norman Lamb, could vote for the deal. If we assume 18 of them won’t, that’s 313 against, leaving Johnson’s deal dangerously close to defeat.

What about looking at it from the other end – how many MPs will support it?

There are 287 MPs who vote as Conservatives. Among them, the most important group is the 28 self-styled “Spartans” – the hard Brexiteers who voted against Theresa May’s deal on all three occasions it was put before Parliament. Their ringleader, Steve Baker, has said the deal is “tolerable” and seems likely to vote for it. Many of the others will take their cue from him. But some of this group is loyal to the Democratic Unionist Party and will face an agonizing decision.

Former home secretary Amber Rudd is likely to support Johnson's deal.

Then, there are the 21 former Conservative MPs who were forced out after voting in favor of the “Benn Act” – legislation that prevents a no-deal Brexit. Johnson can count on the support of most, if not all, of this group, which includes former home secretary Amber Rudd, the longest serving MP in the House of Commons Kenneth Clarke, and former chancellor Philip Hammond. One of their number, Winston Churchill’s grandson Nicholas Soames, told the BBC on Thursday night that he was on board, and he believed most of the others were too.

So let’s be optimistic and put the number of current and former Conservatives in Johnson’s corner at around 305. Add to that the handful of Labour MPs who might support the government, and Johnson gets closer to 310.

Then there’s a ragtag bunch of independents – including some former Labour MPs who owe little to their former leader, Jeremy Corbyn. If they back the deal, Johnson inches closer to that crucial 320.

And what about the Prime Minister’s brother, Jo Johnson? He rejected May’s deal three times because he was in favor of a second referendum. Will family loyalty count for anything on Saturday?

Conservative MP Jo Johnson, the UK Prime Minister's brother, voted against May's deal three times but could he back his brother's deal?

As the hours tick down towards Saturday’s vote Johnson’s fixers will be in overdrive, trying to convince the undecideds, of whom there seem to be anything between a dozen and two dozen. Members of this group can expect to receive a lot of attention from government whips on Friday and Saturday.

Boris Johnson started his premiership with an unprecedented string of seven defeats in a row in Parliamentary votes. Saturday’s vote will arguably be his most important yet. And as it stands, the result is too close to call.