Jim O'Callaghan has said Fianna Fáil should highlight the benefits of a United Ireland to secure the support of young voters.
In his first comments since turning down a ministry offered to him by Taoiseach Micheál Martin, the Fianna Fáil TD said there is a cohort of young voters on the island of Ireland who are not "green or orange" but who see what he calls the "illogicality of partition".
Writing exclusively in the Irish Independent, Mr O'Callaghan added that Brexit negotiations and the Covid-19 crisis have "underlined the benefits of unity".
"As the Republican Party, we must sell this message to all young people on the island," he said.
Mr O'Callaghan said he turned down Mr Martin's offer of a ministry because he wanted to focus on making Fianna Fáil a "radical centre ground national party" which is attractive to young people.
He warned of the "increasing polarisation" of politics in this country which he said will "subdivide people into antagonistic groups".
"Unfortunately, we now see a political landscape in this country where Fine Gael has placed itself on the right and Sinn Féin has placed itself far on the left, each representing their own interest groups," he said.
"I believe the growth of a national centre-left party, such as Fianna Fáil, that can attract the support of young people is the most effective way to confront the increasing threat of polarisation," he added.
Mr O'Callaghan also warned Fianna Fáil will have less influence in Brexit negotiations and Northern Ireland matters after Fine Gael TDs were appointed as Ministers for Justice and Foreign Affairs.
He said it was a mistake to give the two key ministries to Fine Gael.
The Dublin Bay South TD said Brexit negotiations will "loom large over our political landscape" in the coming months and will have "significant consequences for all of the island".
He predicted the UK's exit from the EU will have "unforeseen problems" for people living in Northern Ireland.
"In discussions with the United Kingdom government on Northern Ireland, our ministers for Justice and Foreign Affairs have always played a central role," he said. "However, since Fine Gael occupy both these portfolios Fianna Fáil will not have a senior minister in any such negotiations."
He said Mr Martin will have "infrequent meetings" with the British Prime Minister Boris Johnson, but said detailed negotiations will be "carried out by Fine Gael ministers alone".
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