The National Executive of the Social Democrats has unanimously decided that its Oireachtas members will nominate Catherine Connolly for President.
It said in a statement that the Independent TD for Galway West has been a "tireless advocate for progressive causes and we believe she would make an excellent President".
They said the decision reflects the "strong support throughout the Social Democrats for nominating Catherine Connolly".
Ms Connolly confirmed to RTÉ News yesterday that she will formally declare her candidacy next week.
She said that the campaign was already "taking shape".
Independent candidates can also secure a nomination if they get the support of four local authorities.
The party's deputy leader Cian O'Callaghan said he was "delighted" that Ms Connolly will be running for President.
Social Democrats TD Jennifer Whitmore said Catherine Connolly is an "incredible left-wing politician" and would make an "incredible president".
Speaking on RTÉ's Today with Claire Byrne, she said it is her understanding that as well as the Social Democrats, Solidarity-People Before Profit and a number of independents will back Deputy Connolly.
"I'm not 100% sure as to whether she has the full 20 yet but she is an incredible left-wing candidate and I would really love to see the other left-wing parties coming behind her as well. That would certainly secure that position for her," she said.
Deputy Whitmore said the Social Democrats contacted its elected representatives and its membership and the "overwhelming majority" suggested Catherine Connolly.
"Catherine was the name that just came up again and again and again, and I just think that's a real testament to the politician that she is and her left-wing politics and her integrity that we want to support her," she said.
"I think it would be really good if the other left-wing parties did support her. I think that would obviously make her position a lot stronger and her campaign a lot stronger, but we'll see how that goes," Deputy Whitmore added.
'Process still ongoing' on unified left-wing candidate
Gary Gannon, Social Democrats TD, described Ms Connolly as "an extremely effective parliamentarian" both in the Daíl and in the committees.
On whether councillors will be instructed to support her, he said there has already been favour put towards supporting her from feedback from councillors, but that it was too early to say if they will be instructed to do so.
On whether other left-wing parties will support Ms Connolly, he said that the process is still ongoing and it is for other parties to decide. He added that he cannot decide what Sinn Féin will do.
Speaking to News At One, Deputy Gannon said: "Catherine Connolly is a strong candidate in her own right. I'm just eager now to get behind her and support her campaign."
"I hope Catherine is a unified left candidate. I think that's what a lot of people who have contacted our offices are looking for us to support her," he said.
Larger parties yet to confirm possible candidates
Former government minister Mary Hanafin had previously said that she would love the opportunity to run for President as a Fianna Fáil candidate.
"I would like to bring 30 years of public service, at local and national level, to the job," she said.
Fianna Fáil TD Niall Collins said his party's position would be clear in September.
"I'm open minded in relation to what the party does," he said, adding that he and his party colleagues will take their time and consider all of the options.
On Tuesday, the deadline for interested Fine Gael candidates will close, and in all likelihood Mairead McGuinness will be the sole nomination.
Yesterday, Fine Gael MEP Seán Kelly ruled himself out of seeking his party's nomination for the presidency.
Speaking on RTÉ's Morning Ireland, Mr Kelly said he had given it a lot of consideration but will not be running, adding that it was the "most difficult decision" of his political life.
Sinn Féin said it has "not ruled out" a presidential run for its leader Mary Lou McDonald, while former RTÉ broadcaster Joe Duffy said he is not considering a run for the Áras.
