LONDON — Boris Johnson will finally get a vote on his Brexit deal Tuesday — but that doesn't mean he's home and dry.

After two false starts in which the British prime minister was thwarted in his attempts to demonstrate MPs back his exit agreement, the House of Commons will vote on the deal Johnson brought back from Brussels last week, as his government attempts to push legislation required to turn the agreement into U.K. law through the entire parliamentary process in just a matter of days.

The ambitious timetable — which would see an initial vote on the legislation on Tuesday night with the Commons process wrapped up by Thursday when the bill then passes to the House of Lords — leaves little time for scrutiny as Johnson strives to meet his self-imposed deadline of delivering Brexit by October 31.

Several hurdles stand in his way: First, MPs could refuse to back the bill itself on Tuesday. They could also oppose the unusually short timetable, with a vote on that planned for later the same day.

Even if Johnson wins both of Tuesday's votes, MPs could then attempt to attach amendments to the bill, with opponents signaling that either a second referendum or a requirement to stay in the EU's customs union might gather support from a majority in the Commons. Either would wreck Johnson's plan and instead renew the government's push for a general election.

"I hope parliament today votes to take back control for itself and the British people and the country can start to focus on the cost of living, the NHS, and conserving our environment," Johnson will tell MPs ahead of Tuesday's vote, according to a press briefing.

"The public doesn’t want any more delays, neither do other European leaders and neither do I. Let’s get Brexit done on 31 October and move on."

The Labour Party's Shadow Brexit Secretary Keir Starmer told MPs on Monday that the government is “trying to bounce MPs into signing off a bill that could cause huge damage.”

"The truth is Boris Johnson knows that the more time people have to read the small print of his deal, the more it will be exposed for the risks it represents to our economy and communities across the country,” he said.

The Scottish National Party's Westminster leader, Ian Blackford, echoed the sentiment, saying it is "an insult to democracy that the Tory government is trying to push this bill through on limited time."

For now, the EU is holding its breath, but if word comes that leaders in Brussels will grant an extension to negotiations, opposition MPs will see little incentive to stick to Johnson’s timetable.

Downing Street plan

Downing Street officials are still hopeful they will get the numbers to push the deal through.

Michael Gove, the minister in charge of no-deal preparations, said Sunday that he could “guarantee” Brexit would be done in time. “We have the means and the ability to do so,” he told Sky News' Sophy Ridge.

On Monday, he attempted to drive home the consequences of failure, announcing to MPs that he had triggered Operation Yellowhammer, the government's no-deal contingency plan, and officials would now meet seven days a week to discuss no-deal preparations.

Johnson has the support of most of the hard-line Brexiteers in his own Conservative Party, including Steve Baker, Owen Paterson and Anne Marie Morris. He also has secured the votes of several Labour MPs, including Melanie Onn, Gareth Snell and Sarah Champion. None of them voted for former Prime Minister Theresa May's deal, which was defeated by 58 votes on the third attempt.

But like May, Johnson is missing the support of the Conservative Party’s informal partners in government, Northern Ireland's Democratic Unionist Party (DUP).

The DUP has been resolute in its opposition to the deal, because it says it creates a divergence in rules and regulations between Northern Ireland and the rest of the U.K.

In a House of Lords committee hearing on Monday afternoon, a slip-up by the Brexit Secretary Stephen Barclay was a gift to those making this point.

Asked whether Northern Irish businesses would have to fill out customs declarations when sending goods to Great Britain, Barclay initially replied: “We’ve said in terms of [trade] from NI to GB that it will be frictionless, and so there wouldn’t be [declaration forms].”

But a few minutes later, he interrupted the hearing to read out a correction: “The exit summary declarations will be required in terms of NI to GB.”

Exit summary declarations are required when goods leave the EU’s customs territory. The implication is that the exporters in Northern Ireland would have to complete paperwork in order to sell goods within their own country, the U.K.

The DUP’s Brexit spokesman Sammy Wilson did not pull his punches. “Clear breach of U.K. government commitment in Joint Report of 2017 [negotiated by May] to allow unfettered access to GB market for NI businesses,” he tweeted. “How can any Conservative & Unionist MP argue this does not represent a border in the Irish Sea?!”

The slip-up served to demonstrate the complexity of what MPs must consider in the starkest of ways: The U.K.’s own Brexit secretary could not accurately recall the details of the deal’s implications.

Throughout the day Monday, MPs and peers expressed serious concerns at being asked to rush through such a momentous piece of legislation. Barclay’s response was that they would have time to scrutinize it during the Brexit transition period — after the bill had passed.

One day at a time

Even if the bill survives its first Commons vote with a narrow majority, it could yet be sunk on Wednesday.

Any majority that exists for the proposal is fragile, made up of disparate factions of MPs, who only agree that some form of Brexit should occur.

Once amendments can be proposed to the legislation on Wednesday, the government faces the threat of a customs union or even a second referendum being appended to the deal.

Gloria De Piero, a Labour MP who has expressed her intention to vote for a deal and announced she is standing down at the next election, has tweeted that she will be backing a customs union amendment.

When MPs held non-binding indicative votes in April on a series of possible Brexit outcomes, a customs union came the closest to getting a majority in the Commons. It was defeated by just three votes — 276 to 273 — with a number of abstentions.

Those who voted in favor of a customs union included most of the parliamentary Labour Party, as well as 36 Tories — several of whom have since been expelled from the party, such as Ken Clarke, Alistair Burt, Margot James, Anne Milton, Oliver Letwin, and Stephen Hammond. The Liberal Democrats' Norman Lamb also backed the idea in the indicative vote.