The Labour party spokesman on justice was among those who opposed the merger.
Dr Pat Upton TD, who represents Dublin South Central, told the delegates they were at a defining moment in the history of the party. "The Labour party I joined will be no more if the motion before us today is carried. It will be replaced by a new party."
He said his opposition was not based on self-interest or personal electoral concerns. "We in Dublin South Central held the Labour seat on a dreadful day for the party in 1997 when it was a four-seater. I believe we can hold it again when it is a five-seater."
Nor, he added, was he against the merger because Mr Eric Byrne of DL would be his running mate. "The fundamental case made in favour of the merger is that the new party will be bigger than the two old parties from which it is derived. I do not accept this assertion."
Mr Brian Fitzgerald, Meath, a former TD, said: "Today is probably the most difficult day that I have had in my political career. Indeed, the last number of weeks have been extremely difficult for me and for quite a number of close associates of mine. It went from anger, concern and deep hurt to see what we were being asked to do today.
"I thought I would never live to see the day when we would be asked, in essence, to dissolve the Labour party of 86 years and form a new party. I, delegates, feel very, very sore over that particular issue.
"We are being asked to merge with Democratic Left, who, in my view, for many, many years, has absolutely no principles. It went from Sinn Fein, to Official Sinn Fein, Sinn Fein The Workers' Party, The Workers' Party, The New Agenda, and Democratic Left, leaving a trail of destruction in every party it was part of. And even during that time, its deputies jumped the bannisters in Leinster House to vote for Charlie Haughey as Taoiseach. It did a deal with the Progressive Democrats to get senators elected."
Mr Fitzgerald said a decision on the merger should be postponed until after the next general election. Addressing the party leader directly, he said: "What is being proposed has the potential to tear the party asunder."
Mr Liam Kavanagh, Wicklow, a former TD and minister, said Labour was abandoning its efforts to win seats which obviously could be won again as in 1992.
He said that in the last government, Ms Liz McManus, of DL, who would be the Labour deputy for Wicklow after the merger, was a minister of state with responsibility for housing. "It was then that Labour lost its seat. I can tell you, too, that the worst housing crisis in any constituency or county in Ireland is in Wicklow a year-and-a-half later.
The Dublin Labour MEP, Ms Bernie Malone, who abstained in the vote, recalled that the party had met at the same venue in 1992 and voted to go into government with Fianna Fail.
"In hindsight, if we had a more balanced debate, instead of the overwhelming stampede into the decision, it would have been better for democracy in the party, even if there was no avoiding the inevitable wrath of the electorate. That ill-fated decision eventually lost us half of our parliamentary party."
To loud applause, she added: "I want to be the first Labour MEP to be re-elected. I committed myself full-time to the position when I fought for the seat which I won, against the odds, in 1994."
She said that she had been unanimously re-selected by the Dublin members to contest next year's European elections as the party's sole candidate. "I consider the one-candidate strategy to be the correct one." She added that as a loyal member she would abide with whatever decision was reached by the conference.