LONDON — British Prime Minister Theresa May made a frantic last push Monday to swing lawmakers’ support behind her seemingly doomed Brexit deal, warning that its defeat risked scuttling the U.K.’s departure from the European Union and “betraying the vote of the British people.”

May claimed to have gotten reassurances with “legal force” on key issues from the EU, and said history books would judge Parliament harshly if lawmakers did not back Britain’s orderly exit from the EU when they vote on the agreement Tuesday.

“Over these next 24 hours, give this deal a second look,” May implored skeptical lawmakers in the House of Commons.

“With just 74 days to go until (Brexit day) the 29th of March, the consequences of voting against this deal tomorrow are becoming ever clearer,” she said.

May said rejecting her deal would lead either to a reversal of Brexit – overturning voters’ decision in a 2016 referendum – or to Britain leaving the bloc without a deal, a course that would damage the country’s economy, security and unity.

But the British leader had few concrete measures up her sleeve, and opposition to her deal remains dauntingly strong. A defeat on Tuesday would throw Brexit plans into disarray just weeks before the U.K. is due to withdraw from the bloc.

Britain and the EU reached a hard-won divorce deal in November, a milestone that should have set the U.K. on the road to an orderly exit.

But the compromise deal has been rejected by both sides of Britain’s EU divide. Many Brexit-backing lawmakers say it will leave the U.K. tethered to the bloc’s rules and unable to forge an independent trade policy. Pro-Europeans argue it is inferior to the frictionless economic relationship Britain currently enjoys as an EU member.

May postponed a vote on the deal in December to avoid a resounding defeat, and there are few signs sentiment has changed significantly since then. A handful of previously opposed legislators have swung behind May’s agreement in the last few days, but they remain outnumbered by those determined to vote against it.

In a bid to win support, May sought reassurances from EU leaders about the deal’s most contentious measure – an insurance policy known as the “backstop” that would keep Britain in an EU customs union to maintain an open border between the U.K.’s Northern Ireland and EU member Ireland after Brexit.

Pro-Brexit lawmakers worry that Britain could be trapped indefinitely in the arrangement, bound to EU trade rules and unable to strike new deals around the world.

In a letter to May published Monday, European Council President Donald Tusk and European Commission President Jean-Claude Juncker offered an assurance that the backstop “would only be in place for as long as strictly necessary.”

They promised that the EU would work quickly to strike a permanent new trade deal with Britain that would render the backstop unnecessary.

But the letter also reiterated the bloc’s refusal to renegotiate the divorce deal. The two men said “we are not in a position to agree to anything that changes or is inconsistent with the Withdrawal Agreement.”

Lawmaker Nigel Dodds of Northern Ireland’s Democratic Unionist Party, which fiercely opposes the Irish backstop, said “nothing has fundamentally changed.”

Facing opposition from pro-Brexit members of her Conservative Party and its DUP allies, May sought to win opposition Labour Party lawmakers’ support by promising that the government won’t try to water down environmental standards and workers’ rights after Brexit.

Some opposition lawmakers’ suspect that the government plans to reduce the protections in a bid to boost the economy after Britain leaves the EU.

Labour leader Jeremy Corbyn was unmoved, condemning May’s deal as a “damaging shambles” and calling for a general election if the agreement is rejected on Tuesday.

If Parliament votes down the deal, May has until the following Monday to come up with a new proposal. So far, May has refused publicly to speculate on a possible “Plan B.”

Some members of Parliament from both government and opposition parties are exploring ways to use parliamentary procedures to wrest control of the Brexit process away from the government, so that lawmakers by majority vote could specify a new plan for Britain’s EU exit.

But with no clear majority in Parliament for any single alternate course, there is a growing chance that Britain may seek to postpone its departure date while politicians work on a new plan.

Without a Brexit deal, Britain faces an abrupt break from the EU, a scenario that economists warn could batter the British economy and bring chaotic scenes at borders, ports and airports.

Conservative lawmaker Dominic Grieve, who is spearheading efforts to unite Parliament to prevent a no-deal Brexit, said a cliff-edge exit from the EU would be “national suicide.”

“The economic damage which it will do to us will be immense, so that the most vulnerable in our society will be those who suffer most as a consequence,” Grieve said.

“I’m not prepared to see that happening,” he added.

Story: Jill Lawless