People Before Profit will not enter into any future coalition with Labour because the party 'betrayed their working-class base.'
The party has written to Sinn Féin, The Social Democrats and Left Independents in the hopes of a left government following the next election.
It comes as it prepares to launch a document explicitly ruling out the possibility of coalition with Fine Gael or Fianna Fáil.
People Before Profit TD Gino Kenny told Newstalk Breakfast why the Labour Party was excluded.
"The Labour Party have been in government, and they have - I suppose - betrayed their working-class base," he said.
"We would not consider them in this arrangement in relation to writing to the political entities in the Dáil at the moment.
"The Labour Party have been ruled out, we would not see them as part of a left-wing [government]".
'We have a big problem'
Deputy Kenny said Labour implemented austerity while in power.
"They are similar [to the Social Democrats], but just historically, in relation to the Labour Party, they've been in coalition with Fine Gael and they oversaw austerity... they implemented it," he said.
He believes all parties should rule out sharing power with Fine Gael and Fianna Fáil.
"I think it's a mistake, on their part, to not rule out any coalition with Fine Gael and Fianna Fáil," he said.
"Fine Gael have been in power since 2011 - 12 years in power - we have the worst housing crisis in the history of the State.
"If you don't rule out forming a government with them, then we have a big problem".
'A country set up as a tax haven'
When it was put to him that the PBP document claims a 'wealthy' elite could use Gardaí and the Defence Forces in a coup against a future left-wing government, he said this has happened elsewhere.
"We've seen throughout history where the wealthy and the elite have been challenged that they will turn [to] violence against an elected government," he said.
"We have a country that is set up as a tax haven – essentially that's what [is] set up.
"When the wealthy and the establishment are challenged, they will challenge any sort of political entity that challenged their status.
"We have seen before and we will see in the future, where there's a government that challenges the establishment, the wealthy - do you not think that they will in some way challenge [the government]?
"It's happened in Chile and other countries," he added.