Finance Minister Paschal Donohoe has said a government involving Fianna Fail and Sinn Fein would be a "coalition of the wreckers and reckless".
Mr Donohoe described Mary Lou McDonald's party as "dangerous", and said he believed it will be put into power by Micheal Martin's "spineless" FF.
Mr Donohoe said there is a "cocktail of risk to jobs, taxpayers, public services" and the economy from such an alliance.
The minister's intervention came after Taoiseach Leo Varadkar was forced to defend Fine Gael TDs who said they are open to working with Sinn Fein in government.
Mr Varadkar said there was a "difference" between his party members saying there are "good people" in Sinn Fein and Fianna Fail TDs saying they would be open to going into coalition with Ms McDonald.
Mr Donohoe claimed there is sufficient support among Fianna Fail TDs to go into coalition with Sinn Fein.
"I believe we could have a government that will be a combination of the dangerous and the spineless," he said.
"Dangerous in that we will have policies from Sinn Fein that could scorch our econ- omy. Spineless because I don't believe Fianna Fail will be able to resist those ideas."
The Fine Gael director of elections said the party will, over the next week, be hammering the point home on doorsteps that there is a huge risk to the country.
"Fianna Fail on their own, I would have grave concerns about. Fianna Fail and Sinn Fein together is a policy cocktail that would be bad for jobs, bad for taxpayers and bad for our ability to improve public services," he added.
Fianna Fail finance spokesman Michael McGrath rejected Mr Donohoe's claims and accused Fine Gael of "leaving the door open to going in with Sinn Fein".
"As each day passes, the Fine Gael campaign becomes more dishonest, more desperate and more duplicitous," he said.
"Our leader and our party have been very clear that in no circumstance will we be having any arrangement with Sinn Fein after the election. We made and kept that promise in 2016 and we have made and will keep that promise in 2020."
Mr Varadkar said he has "nothing personally against individuals in Sinn Fein".
"I worked closely with some of them on Northern Ireland," he said.
"There is a big difference between saying there are people in Sinn Fein who are good people and people who you can work with on the situation in Northern Ireland, and being open to a coalition with them."
The Taoiseach's comments came after he listed the Fianna Fail TDs who previously said they were open to going into government with Sinn Fein during a televised election debate.
He said Mary Butler, Kevin O'Keeffe, Darragh O'Brien and Pat 'The Cope' Gallagher were in favour of a coalition.
Last night, Mr O'Brien said the Taoiseach was "wrong" to suggest he supported going into government with Sinn Fein.
"It's Leo who's cosying up to Mary Lou, not Micheal," he added.
Mr Martin dismissed Fine Gael claims his party is divided on a deal with Sinn Fein and claimed it is Mr Varadkar who cannot be trusted on the issue.
He said the same argument was made by Fine Gael before the 2016 election, and added that Mr Varadkar's comments on Fianna Fail TDs who were in favour of a coalition with Ms McDonald's party were "more electioneering".
"He had some TDs in his own party as well, Jim Daly, Kate O'Connell, who were arguing for a Sinn Fein alliance," he said. "I wouldn't trust Leo in terms of not doing a deal with Sinn Fein.
"I think Leo would do whatever it takes to get himself back in the Taoiseach's position."