Northern Ireland's Democratic Unionist Party, which struck a deal with the Conservatives on Monday aimed at keeping British Prime Minister Theresa May in power, has caused alarm in some circles over its incendiary views and virulent past.
The self-styled "Christian fundamentalist" party has softened its fiery anti-Catholicism and other harsh stances over the years -- it no longer calls for padlocking children's playgrounds and closing cafes and bars on Sundays.
But the party that in 1977 launched the "Save Ulster (Northern Ireland) from Sodomy" campaign still holds tight to what critics call its puritanical views, particularly on social issues such as abortion and sexual equality.
And its negotiations with May's government had prompted warnings in the Republic of Ireland of a disrupted balance of power in Belfast that could in turn upset a delicate peace struck after decades of sectarian conflict in Northern Ireland known as the Troubles.
In mainland Britain, protests have erupted over the DUPs opposition to gay marriage and abortion, as well as many senior members' support for teaching creationism, and a history of links to paramilitaries who fought Catholic nationalists during the Troubles.
The DUP has blocked same-sex marriage in Northern Ireland's assembly five times in recent years, with senior members threatening to leave the party if it ever votes in favour.
"Peter will not marry Paul in Northern Ireland," senior party member Jim Wells said earlier this year.
Some senior DUP members -- many of whom belong to the right-wing, avowedly anti-Catholic Orange Order -- even advocate the literal biblical teaching of creationism in every school at the expense of evolution.
Jon Tonge, a professor of history at Liverpool University who has written extensively about the DUP, has said that while the party has become less dogmatic, it certainly cannot be described as pluralist.
In his 2014 book "The DUP: From Protest To Power", Tonge found that 54 percent of party supporters "would mind a lot" if someone from their family married a person of another religion and 58.4 percent would not want their child to go to a non-Protestant school.
So it was a surprise to many political commentators in 2005 when the party agreed to enter a power-sharing arrangement with its bitter enemy Sinn Fein, once the political mouthpiece of the Irish Republican Army, which fought an armed campaign for Irish unity over three decades.
Although the Belfast assembly appeared to operate with reasonable cordiality for much of a decade, it collapsed spectacularly in January over DUP leader Arlene Foster's involvement in a botched renewable heating scheme.
The clash led Sinn Fein to warn of a breakdown in trust, charging the DUP with "arrogance and a lack of respect" for minorities, particularly Irish nationalists.
"It is disappointing that the deep and overlapping anti-Irish and anti-Catholic bigotry of so many DUP-supporting unionists appears to still play a significant role in Northern life and politics," Andy Pollak, former director of the Centre for Cross-Border Studies, said at the time.
Foster has condemned political violence, but her party has long been criticised for sharing platforms with paramilitaries and for an apparent willingness to endorse armed resistance against perceived attempts to "sell out Ulster".
While the DUP promised to vote in favour of legislation linked to Brexit, it could prove a difficult partner during the negotiations.
The party campaigned for Brexit in last year's referendum but is faced with growing concern in Northern Ireland about the prospect of checks being reimposed the border with the Irish Republic -- a reminder of the bad old days of the Troubles.
In pro-EU circles, that has led some to hope that Foster could moderate May's stance on Brexit.
"The Democratic Unionists have chosen to prop up a government that remains intent on a hard and destructive Brexit," James McGrory, head of the Open Britain campaign, said on Monday.
"It is crucial that they do not betray the voters by going back on their manifesto promises and caving in to ministers' obsession with an extreme and chaotic exit from the EU.