Facing possible defeat in a fateful vote looming in the House of Commons over her plan to leave the European Union, British Prime Minister Theresa May on Monday warned lawmakers they must honor the results of the 2016 referendum and deliver Brexit to the people.

In a speech to workers at a British pottery manufacturer in the town of Stoke-on-Trent, where two-thirds of voters cast ballots to leave the European bloc, May told the rebels in her Conservative Party that they faced a stark choice: either accept her imperfect but doable deal or cut ties with Europe with no deal at all, an option now favored by many of her fellow Tories but which economists predict could inflict chaos and financial pain.

“With no deal, we would have no implementation period, no security cooperation, no guarantees for U.K. citizens overseas, no certainty for businesses and workers here in Stoke and across the U.K,” the prime minister said.

Just an hour after her remarks, one of May’s Conservative Party whips in Parliament, Gareth Johnson, whose job it is to wrangle votes for the prime minister’s agenda, abruptly resigned. Johnson said he could not support May’s half-in, half-out compromise deal, which he said “prevents us taking back control and instead could leave us perpetually constrained by the European Union.” He was the 13th member of May’s government to quit over Brexit.

On the eve of Tuesday’s vote in the House of Commons, lawmakers remained deeply unhappy with the terms of the withdrawal agreement that May spent two years negotiating in Brussels.

The opponents — led by former foreign minister Boris Johnson and former Brexit secretary David Davis — say May’s accord condemns Britain to be a “vassal” of Europe, taking laws but not writing them. The Euroskeptics argue that the 585-page withdrawal agreement keeps Britain too closely aligned with European tariff rules and customs regulations, and forestalls bigger, better trade deals around the world.

President Trump has warned that May’s deal might make it hard to forge a new trade accord with the United States. The U.S. ambassador to Britain, Woody Johnson, suggested the same, saying a “quick, massive, bilateral trade deal” would be blocked by May’s pact. The British prime minister, blindsided by Washington, responded by saying she didn’t agree.

Before making any new deals, May has struggled to extricate Britain from the E.U. She has steadfastly argued that critics such as Boris Johnson and Davis may complain all they like, but they could never get a better offer.

During a speech to Parliament on Monday, May urged skeptical lawmakers to think again.

“Whatever you may have previously concluded, over these next 24 hours, give this deal a second look,” May said. “No, it is not perfect. And yes, it is a compromise. But when the history books are written, people will look at the decision of this House tomorrow and ask: Did we deliver on the country’s vote to leave the European Union?”

In response, the leader of the Labour Party, Jeremy Corbyn, said that nothing had changed since December, when May pulled the vote because it was facing defeat. He called on lawmakers to “reject a deal that is clearly bad for this country.”

Parliament is divided among those who grudgingly support May’s deal; those who want her to try to renegotiate it; those who want to leave now with no deal; and those who don’t want to leave Europe at all but instead are pressing for a second referendum, a do-over, where the British people would be asked again — knowing all they know now — what they really want.

But May warned that “we would risk a subversion of the democratic process” if the Brexit project is abandoned.

“I ask MPs to consider the consequences of their actions on the faith of the British people in our democracy,” she said.

“What if we found ourselves in a situation where Parliament tried to take the U.K. out of the E.U. in opposition to a remain vote? People’s faith in the democratic process and their politicians would suffer catastrophic harm,” she said. “We all have a duty to implement the result of the referendum.”

May again pledged that “we are leaving on the 29th of March” and that she did not want to extend the deadline, as many have urged, by extending or revoking Article 50, which dictates the time table.

But if May loses Tuesday in the House of Commons — and keeps her job and goes back to Brussels to talk more — then it is possible the deadline would be extended.

May long promised fresh assurances from Brussels, and Monday, she presented a letter from E.U. chiefs that she said bolstered her case that her withdrawal agreement was the “best deal possible” and “worthy” of lawmakers’ support.

In their five-page correspondence, European Council President Donald Tusk and European Commission President Jean-Claude Juncker stressed that “we are not in a position to agree to anything that changes or is inconsistent with the Withdrawal Agreement.”

But they did address the thorny issue of the “Irish backstop,” a guarantee that seeks to ensure that there is no hard border on the island of Ireland. Many of May’s critics fear that it could lead to Britain being permanently trapped in an E.U. customs union.

Tusk and Juncker wrote that they did “not wish to see the backstop enter into force,” which would “represent a suboptimal trading arrangement for both sides.” They said the E.U. would work “as quickly as possible” to strike a new arrangement with Britain that would mean the backstop wouldn’t kick in.

May said that the letters she and the E.U. leaders exchanged have “legal force” and “make absolutely clear the backstop is not a threat or a trap.”

But the note didn’t quiet May’s critics, who said the assurances were vague and didn’t go far enough — that essentially the letter was a nothingburger.

“The letter certainly isn’t legally binding,” Nigel Dodds, deputy leader of the Democratic Unionist Party, told the BBC. “The prime minister is going to struggle to justify what the delay was about.”

The DUP is a Northern Irish party that props up May’s minority Conservative government.

The British leader said she knew that the letters “do not go as far as some MPs would like” but added, “I am convinced that MPs now have the clearest assurances that this is the best deal possible and that it is worthy of their support.”