Party sources said that the two active politicians will be left to battle it out as parachute candidate Pat Cox and retiree Avril Doyle fade.
"Many people within the party now see it as a straight choice between the country woman and the city man," one TD told the Herald.
It is understood that some of those who have already backed Mr Cox or Ms Doyle are now reconsidering their vote. Of the party members who have gone public with their support, most appear to be backing Ms McGuinness. But behind the scenes Mr Mitchell has growing support.
All four hopefuls will get a final opportunity to sway colleagues at a party selection convention at the Regency Hotel in north Dublin on Saturday afternoon, with their fate expected to be decided by around 5.30pm.
Between the party's Oireachtas members, MEPs, councillors and members for the Fine Gael executive council, over 600 members are entitled to vote. The electoral college system will be used for the first time in a presidential election with the PR-STV method to be employed in voting.
Fine Gael Oireachtas members and MEPs will count for 70pc of the total poll, while local council representatives will comprise 20pc and the remaining 10pc allocated to executive members.