Progressive Democrats TD Ms Liz O'Donnell should run in the Dublin European Parliament constituency, the party leader and Tánaiste made clear at the weekend.
Ms Harney's enthusiastic declaration was issued following a two-hour special party conference in Dublin to decide on new selection rules for the leadership.
Deputy O'Donnell, a former minister of State, said she would make a decision "imminently" on whether or not to contest the election.
"We will have to have our candidate selected in advance of the party conference at the end of March. I would expect a decision before then," she told The Irish Times.
Her choice could badly hurt Fine Gael's chances of retaining a seat in the capital, following the decision of sitting FG MEP, Ms Mary Banotti, to stand down. So far, the party has not yet found a high-profile replacement.
Asked to outline the factors that she has to weigh up, Ms O'Donnell said: "It isn't really about me. It is about a strategic decision for the party to consolidate our base in Dublin by having an MEP. We haven't had an MEP since Pat Cox."
Asked if she wanted Ms O'Donnell to run, the Tánaiste said: "Of course, I want a winner." Asked if there was any reason why the Dublin South TD should not run, Ms Harney replied: "None whatever. She has a huge interest in foreign policy issues, a good track record. She would be a winning candidate."
However, Ms O'Donnell expressed "concern" that some people would interpret a decision by her to run for the European Parliament elections in June as a declaration that she was withdrawing from domestic politics.
