LONDON — After years of debate, false starts and just plain confusion, the news that Britain had reached a deal to leave the European Union seemed like a moment for a weary nation to exhale.
But nearly as soon as Prime Minister Boris Johnson announced news of the deal, there was a problem.
The Democratic Unionist Party of Northern Ireland, a group of lawmakers whose support has long been viewed as essential to win passage of any Brexit deal, said it could not back the draft agreement.
The opposition of the Northern Irish lawmakers raised questions about whether Mr. Johnson thought he had enough votes without them to push the plan through Parliament in a special session scheduled for Saturday.
For those not steeped in every twist and turn of the seemingly endless Brexit drama, it raised a more basic question: How could a party that holds only 10 seats out of 650 in the British Parliament wield such influence over Brexit?
The Democratic Unionist Party was founded in 1971 as a radical, hard-line Protestant political faction during the Troubles, the 30-year sectarian conflict that began in 1968.
The conflict was mainly fought between Catholics who wanted a republic that encompassed all of Ireland and Protestants determined to keep Northern Ireland in the United Kingdom.
At least 3,532 people, most of them civilians, lost their lives to paramilitary killings and terrorist bombings, with the violence at times spilling over into England and the Republic of Ireland.
The D.U.P.’s founder, the Rev. Ian Paisley, was an evangelical preacher whose virulently sectarian speeches, and sometimes violent demonstrations, helped stoke interfaith tensions in the early years of the Troubles.
Years later, the party was a main beneficiary of the Good Friday Agreement of 1998, which ended the Troubles. The deal stipulated that the largest Protestant and Catholic parties should share power in Northern Ireland. But it soon became apparent that this was pushing voters from both sides to the political extremes.
By 2017, voters had largely abandoned moderate parties and the Democratic Unionists took the last Westminster seats held by the rival Ulster Unionists. At the same time, Sinn Fein, the political wing of the disbanded Irish Republican Army, eclipsed more moderate nationalist parties.
Sinn Fein, which does not consider itself British and has formally renounced any involvement in Westminster politics, refuses to vote in the House of Commons, so has not played a role in the Brexit fight.
When Boris Johnson’s predecessor as prime minister, Theresa May, was riding high in the opinion polls in 2017, she decided to call an election to try to cement a strong majority to get her Brexit plan through Parliament. As it happened, she fared poorly, actually lost ground and became dependent on the D.U.P.’s 10 votes to stay in power.
That gave the unionist party an outsize role in the Brexit negotiations, which it used to enforce its bedrock position rejecting any plan that would divide Northern Ireland from the rest of Britain.
“The red line is blood red,” Arlene Foster, the party leader, said last fall as discussion swirled about a possible compromise.
“There cannot be a border down the Irish Sea, a differential between Northern Ireland and the rest of the U.K.,” she told the BBC at the time.
So it was not a surprise that the party put out a statement Thursday morning warning that “as things stand, we could not support what is being suggested on customs and consent issues.”
In addition to providing the Conservative government with critical votes in Parliament, the D.U.P. also acts as a guidepost for many hard-line supporters of Brexit, who have said that they will never vote for a deal that does not also have the Northern Irish party’s support.
In the statement early Thursday, before a deal had been reached, the D.U.P. seemed to leave a door open for concessions, saying it would continue to work with the government to find a solution.
But as the day wore on, they denounced the deal in even more strident terms.
“We have been consistent that we will only ever consider supporting arrangements that are in Northern Ireland’s long-term economic and constitutional interests and protect the integrity of the union,” the party said, referring to Northern Ireland’s ties to rest of the United Kingdom.
“These proposals are not, in our view, beneficial to the economic well-being of Northern Ireland and they undermine the integrity of the union,” the party added.
It was far from clear how Mr. Johnson would convince them to change their minds.
He has until Saturday.