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Chalk up a big victory for Boris Johnson in Brussels: the 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 MPs who do vote, 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 they 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. Let’s figure around 240 Labour MPs will vote against – although Johnson will actually be hoping as many as two dozen will swing to his side.

The Democratic Unionist Party, which props up the Conservative government, says they won’t vote for the deal. There are only 10 of them, but they’re a disciplined bunch with very clear views, 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, one of whom 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.

But could he corral all of the remaining lawmakers to back him so he squeaks out a 326-313 victory?

Most of the Conservative Party will back the deal, as will many of the independents who were Conservatives until Johnson kicked them out of the party for voting against him in the past. The optimistic estimates put the number of current and former Conservatives in Johnson’s corner around 305. But that leaves between a dozen and two dozen votes that could go either way, even without a significant rebellion from the Conservatives who call themselves the European Research Group and are hardline Brexiteers.

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.