Arlene Foster has insisted that her party still has influence at Westminster after the Prime Minister reached a deal with Brussels despite strong DUP opposition.
As MPs prepare for a knife-edge vote on Boris Johnson's agreement with the EU, Mrs Foster said the votes of the DUP's 10 MPs would be crucial.
But Boris Johnson said he was "very confident" the Commons would back his deal in tomorrow's vote despite the DUP's position.
There will be intensive behind-the-scenes efforts over the next 24 hours to win over ERG members, Tory rebels who lost the whip, and Labour MPs in Leave constituencies.
Writing in today's Belfast Telegraph, Mrs Foster said: "Tomorrow's vote will only be the start of a long parliamentary process to pass the Withdrawal Agreement Bill.
"We will continue to work to shape those outcomes in Northern Ireland's interests and our votes will be critical in that process.
"It is only as a result of the strong mandate that we received at the last election that we are able to continue to have a critical role in the House of Commons."
The DUP leader claimed that advances had been made in the Brexit negotiations.
"There was progress. Dublin and Brussels belatedly reopened Mrs May's deal having rigidly refused to do so," she said.
"But the Prime Minister was under pressure and conceded too much by way of agreeing to greater East-West checks than necessary, and settling for a one-sided consent mechanism."
Sinn Fein gave a cautious welcome to the deal. Speaking after meeting Tanaiste Simon Coveney in Dublin, Mary Lou McDonald said she was satisfied that there would be "no hard border on the island of Ireland and no veto will be given to the DUP".
It is proposed that Northern Ireland's post Brexit trading arrangements will be decided by a straight majority vote in the Assembly, which enjoys a pro-Remain majority.
But the Sinn Fein president added: "There is no such thing as a good Brexit.
"Brexit is being foisted on the north of Ireland against the democratic wishes of the people. As a party, Sinn Fein has worked to defend Irish interests from the worst impacts of Brexit."
Ulster Unionist leader Robin Swann said the deal was disastrous for unionists and he laid the blame at the DUP's door.
"It's an absolute disgrace that Northern Ireland has been led to the place it's in today. This deal is worse than the one Theresa May brought forward at Chequers. It's awful," he said.
"It would put a customs and regulatory border down the Irish Sea and annexe Northern Ireland from the rest of the UK."
Mr Swann added: "We warned the DUP that their acceptance of a border in the Irish Sea would open the floodgates and so it has come to pass.
"Their proposals were and are a serious threat to the Union. By accepting the principle of a border in the Irish Sea, they opened unionism up to exploitation by Dublin and Brussels."
SDLP leader Colum Eastwood said that while the agreement was by no means ideal, it should prevent a hard border.
"This confirms our long-stated position that there is no such thing as a good Brexit - only degrees of harm caused to economic and political stability on this island," he said.
"This deal does however meet the minimum requirements to avoid a hard border and the introduction of customs or regulatory checks across this island. If the choice is between this deal or no-deal, then the interests of people here must be defended."
He urged the DUP to think carefully before making its next move. "Opposing this deal in the absolute terms they have suggested could mean a no-deal Brexit that would devastate this island, particularly in the farming and agricultural sector," he said.
"If they don't like what's on the table, then maybe they shouldn't back Brexit at all."
Alliance leader Naomi Long said a second referendum was the only choice between a no-deal and a poor deal.
"It certainly appears to be a worse deal than the original backstop," she said. "It is not a solution which provides long-term economic or political certainty, with businesses left in a flux and any restored Assembly put under continued pressure.
"There is no such thing as a good or sensible Brexit, but we recognise no-deal will be catastrophic for Northern Ireland in particular. People were promised unicorns in 2016, yet now they are now being asked to choose between a lame donkey and a dead donkey."
TUV leader Jim Allister said the scale of betrayal show by the new Brexit deal was "shocking".
"This is a disastrous deal for Northern Ireland's position as an integral part of the UK and for our economy, particularly as the rest of the UK can move on to thrive outside the EU, but not Northern Ireland," he said.
"While Great Britain can diverge and embrace the full opportunities of Brexit, Northern Ireland is left behind in the clutches of EU/Republic. The scale of the betrayal is shocking."
Green leader Clare Bailey said she'd rather see a referendum. The proposal around a vote in a defunct Stormont Assembly is paper thin. I'd rather see a say for the people of Northern Ireland as a whole," she said.