Sinn Féin has become the biggest party in Northern Ireland local authorities after fantastic successes in council elections.

It swept past the Democratic Unionist Party (DUP) and became the first nationalist party to hold the most council seats, dealing a political and psychological blow to the trade union movement.

Sinn Féin made unprecedented inroads in parts of County Down and County Antrim and almost doubled its vote from the 2019 local elections in Balmoral, Belfast’s most affluent ward.

With 456 of 462 seats spanning 11 councils declared on Saturday night, Sinn Féin had 143, up from 105, and the DUP remained on 122. The Non-Aligned Alliance had 65, a gain of 14, while the moderate Ulster Unionist Party (UUP ) ) and the moderate nationalist Social Democratic Party and Labor Party (SDLP) fell to fourth and fifth place respectively.

The share of first-place votes underlined Sinn Féin’s dominance, winning 30.9% to 23.3% for the DUP, 13.3% for the Alliance, 10.9% for the UUP and 8.7% for the SDLP.

Nationalist parties won 19,000 more first preference votes than unionist parties, according to Nicholas Whyte, a psephologist at consultancy Apco Worldwide.

The election was also notable because Lilian Seenoi-Barr, an SDLP candidate in Derry, became the first black person to be elected as a politician in Northern Ireland.

Sinn Féin tapped into nationalist resentment over a DUP boycott of power-sharing which has blocked Sinn Féin’s deputy leader, Michelle O’Neill, from becoming First Minister. O’Neill led a presidential-style campaign that attracted SDLP voters to support Sinn Féin, which was an IRA mouthpiece during the Troubles.

“These election results are a positive endorsement of Sinn Féin’s message that workers, families and communities need to be supported and that the blocking of a new assembly by one party must end now,” said O’Neill, who was mobbed by cheering supporters.

The DUP leader, Sir Jeffrey Donaldson, became a recruiting sergeant for republicans, Suzanne Breen, a commentator, wrote in Belfast Telegraph. “The longer Michelle O’Neill is blocked from becoming First Minister, the more voters are driven into the arms of her party.”

Sinn Féin’s triumph cemented its success in last year’s general election – when it eclipsed the DUP as the largest party – and presented the trade union movement with another reminder of its shrinking demographic and political standing.

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“For the trade union movement it is perhaps a ‘wake up and smell the coffee’ moment,” Edwin Poots, a former DUP leader, told the BBC.

He said the result showed the need for greater unity among unionists, who split their vote between the DUP, UUP and Traditional Unionist Voice (TUV). Analysts said the result raised deeper, more worrying questions for those who favor keeping Northern Ireland in the UK.

The DUP emerged intact from what was one proxy referendum on his decision to paralyze the Stormont leadership and assembly to protest post-Brexit trade arrangements.

While this herded nationalists into Sinn Féin, it also consolidated the DUP as the dominant force of unionism.

The moderate UUP and the hardline TUV were pressured, allowing Donaldson to claim justification for the DUP policy of claiming credit for the Windsor Framework, which tweaked the original Northern Ireland Protocol, and demanding further concessions before restoring the institutions of the Good Friday Agreement. There is speculation that the DUP will do so in the autumn.

The Alliance consolidated its position as Northern Ireland’s third largest party. Lewis Boyle, an 18-year-old A-level student who stood for the party in Antrim and Newtownabbey councils, became what was believed to be Northern Ireland’s youngest elected representative.

Colum Eastwood, the leader of the SDLP, said he would not resign, despite the party’s losses. “If I thought the right course of action was to resign, I would do it in a heartbeat,” he said.