Fine Gael leadership contender Simon Coveney has said that some ministers who pledged support to him had switched sides over recent days.
Mr Coveney said these included some members of the Cabinet.
He said he believed he was assured of support from more than the 20 members of the parliamentary party who have declared for him.
Leo Varadkhar, his rival for the leadership, currently has the support of 45 Fine Gael parliamentary members.
The two men are bidding to succeed Enda Kenny who stood down as the party’s leader during the week, but will stay on as prime minister until his successor is chosen.
“I would not have spent the last few years in politics if I was afraid of big challenges. I’m not. Declarations are not votes,” Mr Coveney said
Nominations for the contest closed at 17:00 on Saturday, with both candidates opening their campaigns on Sunday.
Asked how Mr Varadkar had been so certain of the strength of his support, Mr Coveney told RTE: “Well, he wasn’t actually, because from some of the people I have spoken to, who would have been quite supportive, a number of days ago, never mind weeks ago, you know they’ve changed their mind.
“People make decisions in politics for all sorts of reasons and I’m not going to start judging people. My job now is to put a persuasive argument together over the next 10 days to change the mind of some of those,” he said.
“When leadership contests are under way some people who know and like both candidates want to get themselves on the winning side. And I think there was a bit of that over the last few days. But I don’t blame any colleagues for that,” he added.
Mr Coveney said that while he accepted the campaign was a personality contest, it was also a fight for the soul of Fine Gael.
Mr Varadkhar said he was pleased that Mr Coveney had decided to stay in the leadership contest, saying “it’s absolutely important that we take this debate and this opportunity to reinvigorate Fine Gael to the councillors and members across the country”.
He said he will be publishing some policy proposals on Monday and a document on how Fine Gael can be modernised and energised during the week.
He said his campaign was going very well and he is feeling very confident but taking nothing for granted.
Over the last year, Mr Kenny led a minority government, which was propped up by an alliance of independent TDs and required the support of the opposition party – Fianna Fáil – to pass its budgets.
Mr Kenny leaves the Fine Gael leadership as the party’s most successful taoiseach.
In a statement announcing his retirement, he said it had been a “huge honour and privilege” to lead the party over the course of 15 years.