Jim O'Callaghan has said Fianna Fail 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 Micheal Martin, the Fianna Fail 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".
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 had turned down Mr Martin's offer of a ministry because he wanted to focus on making Fianna Fail 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 Fein has placed itself far on the left, each representing their own interest groups," he wrote in the Irish Independent.
"I believe the growth of a national centre-left party such as Fianna Fail that can attract the support of young people is the most effective way to confront the increasing threat of polarisation."
He said people aged between 25 and 45 have now experienced two recessions in their adulthood.
"Most live in a world of economic insecurity where affordable accommodation is beyond their means, and terms of employment are now much more insecure than was the case when their parents first started working," he said.
"Climate change and Covid-19 have imposed even further burdens upon them - burdens they carry for the benefit of older and future generations."
Mr O'Callaghan also warned Fianna Fail 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" and will have "significant consequences for all of the island".
He predicted that 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 Fail will not have a senior minister in any such negotiations."
He said Mr Martin will have "infrequent meetings"with the UK Prime Minister Boris Johnson but said detailed negotiations will be "carried out by Fine Gael ministers alone".
"The country and Government would have benefited had these portfolios been divided between the two main parties of Government," Mr O'Callaghan added.