Theresa May takes her Brexit publicity blitz to Scotland on Wednesday, as opponents of her deal scramble to plan for the chaotic aftermath of the meaningful House of Commons vote in less than a fortnight, which she appears likely to lose by a crushing margin.

The prime minister will visit a factory near Glasgow and speak to workers and employers about the agreement, telling them: “It is a deal that is good for Scottish employers and which will protect jobs.”

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Back in Westminster, few MPs believe the controversial package is likely to pass the Commons, despite a charm offensive from Tory whips, who are pressing the argument that none of the alternatives – from a Canada-style trade deal to a second referendum – could command a majority.

Jeremy Corbyn’s team is planning for a range of possible scenarios as the Labour leader prepares to ramp up efforts to explain his alternative plan to the public in the coming days. Corbyn is expected to confront the prime minister in a head-to-head television debate.

The Labour leadership is determined to reject the idea gaining ground at Westminster of a Norway-plus deal. The Scottish first minister, Nicola Sturgeon, said on Tuesday she hoped a majority could coalesce around the proposal, which is being promoted by the Conservative backbencher Nick Boles.

However, senior Labour party sources insisted they regarded it as an unacceptable abrogation of sovereignty that would fail to honour the referendum result, won by the Vote Leave campaign with the slogan “take back control” – though some shadow cabinet members will be keener to explore it.

Sturgeon said the alternative framework for the UK, which the Scottish government first advocated in December 2016, “may have met with a disinterested rejection from the UK government but … is gathering support from others. The arguments that the Scottish government has been making are actually winning the day.”

Almost half of May’s cabinet have held talks to weigh up the possibility of backing the Norway-inspired alternative, in which the UK could join the European Free Trade Association and maintain a customs arrangement with the EU if parliament rejects the prime minister’s EU withdrawal deal next month.

But Labour intends to hold out for its own plan, involving a permanent customs union and a close relationship with the single market that falls short of full membership. It will resist demands for a second referendum unless all other options have been exhausted.

Fresh ammunition for MPs hoping to halt Brexit altogether may come from the government’s economic analysis of the deal, which will be published on Wednesday and is expected to echo the findings of a leaked assessment from earlier this year that showed GDP would be hit.

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The government was keen to stress the report, prepared by civil servants across Whitehall, constituted an “analysis” and not a “forecast”.

Sturgeon said the Scottish National party would support either the Norway plan or a second referendum, whichever appeared most likely to command a majority in the Commons.

Leave-leaning Tories are hoping a harder Brexit could emerge if the deal fails to pass the Commons. Several cabinet ministers, including Penny Mordaunt and Andrea Leadsom, have not yet signalled their public support.

The prime minister’s prospects of winning the vote on 11 December appear increasingly bleak, with 94 of her MPs pledged to vote against her.

May was forced to defend her plan against hostile comments from the US president, Donald Trump, on Tuesday. He said the agreement “sounds like a great deal for the EU” and warned “they [the UK] may not be able to trade with us”.

Trump’s intervention came as No 10 attempted to marshal support from MPs in the face of a chorus of condemnation at Westminster. Conservatives were briefed on the deal on Tuesday evening by May’s chief of staff, Gavin Barwell.

Speaking in Wales on the first leg of a UK-wide tour to promote the deal directly to the British public, the prime minister said: “We will have the ability, outside the European Union, to make those decisions on trade policy for ourselves. It will no longer be a decision being taken by Brussels.

“As regards the United States, we’ve already been talking to them about the sort of agreement we could have with them in the future.”

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However, Peter Mandelson, a former EU trade commissioner, offered unlikely support for the president’s view, declaring “Donald Trump is right”. The former Labour minister warned in an article for the Guardian that it may not be possible to sign a trade deal with the US until the end of 2022, the last possible date for the end of the post-Brexit transition period, with the UK tied to EU trade rules before that.

Hard Brexiters who dreamed of the UK becoming a “North Sea Singapore” through deregulation would be disappointed, Mandelson said, because the political declaration signed by May with the EU “agrees that Britain would closely follow EU standards and rules affecting competition”.

Trade deals, Mandelson concluded, were the product of years of negotiation. “It took Donald Trump to remind the UK government that negotiating trade deals is a long, painful and necessarily complex business,” he wrote. “I can hardly believe I am writing this sentence … Donald Trump is right.”

Before May arrived in Northern Ireland on the second leg of her tour on Tuesday, the Democratic Unionist party leader, Arlene Foster, renewed her criticism of the Brexit deal, telling the BBC: “The disappointing thing for me is that the prime minister has given up and she is saying … we just have to accept it.”