The Democratic Unionist Party (DUP) has said it is “encouraging” Conservative MPs to vote down Boris Johnson’s new Brexit deal on Saturday.
The UK and EU announced a revised plan that includes different proposals to avoid a hard Irish border.
But the DUP will not support it and has claimed the deal is not in Northern Ireland’s best interests.
Its backing is seen as crucial if the government is to pass a deal in the Commons by the 31 October deadline.
MPs are due to decide on the deal during a special sitting of Parliament on Saturday, in what is expected to be a knife-edge vote.
Prime Minister Boris Johnson has said he is “very confident” MPs will back his deal.
Sammy Wilson, the DUP’s Brexit spokesperson, said his party was talking to a range of Conservative MPs in a bid to vote down the plan.
Mr Wilson told Radio 4’s Today programme that the DUP was encouraging those Tory MPs to “take a stand” with them.
A number of Conservative MPs in the Eurosceptic European Research Group (ERG) have supported the DUP’s stance on Brexit in the past – but it is not clear if any of them will vote against the government this time.
Mr Wilson added that the DUP was “always prepared” for the possibility that the prime minister might turn his back on working with the party.
Speaking on BBC Radio Ulster he said the party had not spoken to Mr Johnson since Tuesday.
“He’s knows us well enough to know that we will not sell Northern Ireland short in the way in which this deal sells Northern Ireland short,” he said.
“So that is probably the reason he hasn’t come to us.”
On Thursday, DUP deputy leader Nigel Dodds said Boris Johnson had been “too eager to get a deal at any cost”.
Mr Wilson argued that voting down the deal in Parliament was not the end, and would “open up the possibility” for the government to call a general election and win an overall majority.
If the government loses Saturday’s vote, the prime minister is legally required to ask the EU for another extension to the Brexit deadline.
What does the deal involve for NI?
The new Brexit deal would involve Stormont giving ongoing consent to any special arrangements for Northern Ireland via a straight majority.
Pro-EU parties have a narrow majority at Stormont and there would be no unionist veto, as demanded by the DUP.
Northern Ireland would continue to follow EU rules on food safety and product standards and would also leave the EU customs union.
But EU customs procedures would still apply on goods coming into Northern Ireland from Great Britain in order to avoid checks at the border.
Stormont would have to approve those arrangements on an ongoing basis.
Approval would involve a straight-forward majority, which would keep the special arrangements in place for four years.
Alternatively, if the arrangements are approved by a majority of nationalists and a majority of unionists, they would remain in place for eight years.
If the Northern Ireland Assembly voted to end the arrangements there would be a two-year notice period, during which the UK and the EU would have to agree ways to protect the peace process and avoid a hard border.
There is no fallback position in case the two sides cannot find a solution.
If a vote was not held – by choice or because the assembly was not sitting – then the government has committed to finding an “alternative process”.