The Transport Minister says it will be a long road back to the Fine Gael Parliamentary Party for Junior Minister Lucinda Creighton if she votes against the abortion legislation.
The new Protection of Life During Pregnancy Bill will complete all stages in the Dáil later.
Some of the 165 amendments tabled will be debated before the key vote at around 10pm, when all eyes will be on Minister Creighton.
Michelle Mulherin, John O'Mahony or John Paul Phelan may also join her in voting against the Bill.
Transport Minister Leo Varadkar says Minister Creighton will face an uphill battle to get back into the parliamentary party if she loses the whip tonight.
"There's a possibility that they could come back to the party but that's not easy - you'd have to get a vote in the Parliamentary Party to re-admit you" he said.
"You'd have to get your own party members to select you and I think that may be a significant issue for Lucinda in the future, given that she represents a very liberal constituency actually - both in terms of voters and the party membership are very liberal in that constituency" he added.