BELFAST — "Stick to your word!" Northern Ireland's Democratic Unionist Party has been at the heart of Britain's Brexit saga, and its leaders warned on Saturday they would keep up the pressure on Prime Minister Boris Johnson not to sell them out.

Once a fringe faction in the House of Commons, the DUP became a power player when it agreed to prop up the Conservative minority government in 2017 — an alliance that remains under new premier Johnson.

But they are outraged at the Brexit divorce terms he has struck with European Union leaders, warning they threatened Northern Ireland's place within the United Kingdom.

Last weekend, the DUP voted with opposition parties to withhold Commons approval for the deal, forcing Johnson to ask Brussels to delay Brexit beyond Oct. 31.

At the party's annual conference in Belfast on Saturday, DUP leader Arlene Foster noted the party's 10 lawmakers in the Commons had a "critical" role.

"We want to support a deal, a deal that works for the whole of the United Kingdom and which does not leave Northern Ireland behind," she told delegates.

"But without change, we will not vote for the prime minister's agreement."

The DUP's leader in the Commons, Nigel Dodds, urged Johnson to fulfil promises that there would be no new trade barriers between Northern Ireland and mainland Britain.

"Stick to your word, prime minister," he said.

Former Conservative prime minister Theresa May lost her majority in the 2017 election, forcing her to strike a deal with the tiny DUP to stay in office.

The price was £1 billion ($1.3 billion, 1.15 billion euros) in extra funding for Northern Ireland.

The DUP agreed in principle to back the government on Brexit — but then denied votes for May's Brexit deal three times, spurring her exit from office in July.

Johnson renegotiated the deal, but the DUP is still unhappy.

His new terms would keep Northern Ireland in partial alignment with some EU rules and standards, requiring checks on trade crossing the Irish Sea in order to keep the land border with EU member Ireland free-flowing.

"The union for us is the guiding star and we will do everything to protect the union," local DUP lawmaker William Humphrey told AFP at the conference.

The DUP's tough stance puts it in a tricky political spot in Northern Ireland, which voted against Brexit by 56-44 percent in the 2016 UK referendum.

Polls show the party losing up to three seats in an early election sought by Johnson — although Foster insisted her party was "ready for any general election that may come".

At a meeting of unionists and more hardcore loyalists held Monday in Belfast, one pamphlet called Johnson's deal a "betrayal act".

Loyalist spokesman Jamie Bryson reported "immense anger" in the room, adding: "The message to the DUP is to stand firm."

But the DUP's disdain for Johnson's deal may not hold as much sway as it did over May's failed agreement.

On Tuesday, the Commons gave its approval in principle for legislation to ratify Johnson's Brexit deal — without the DUP's support.

MPs also rejected his plan to rush it through before October 31, leading Johnson to pause the ratification process.

Some feel the DUP's pro-Brexit stance puts it at odds with its unionist traditions — and could ultimately be its undoing.

Unless the sides come up with a different solution, Brexit will require some form of checks at British borders when the country leaves the EU's single market and customs union.

All sides agree these cannot be done along the Irish border, which was rendered "invisible" under a 1998 peace accord that end three decades of bombings and bloodshed known as "The Troubles".

The long-running conflict pitted the Catholic IRA, supporting a united Ireland, against Protestant 'loyalist' groups who favored remaining in the United Kingdom.

"The party is right to claim that the new deal will, over time, loosen Northern Ireland's place within the union," commentator Fintan O'Toole wrote in the Irish Times last week. — AFP