Labour TD Willie Penrose has announced he will not contest the next General Election.
The Longford/Westmeath deputy was first elected to the Dáil in 1992, and has held his seat since.
Former Westmeath GAA star Alan Mangan is expected to be selected this evening as the party's next candidate for the constituency.
Deputy Penrose says Mr Mangan has the party's full support.
He said: "I'll be honest with you, I would not be stepping down if I did not feel the candidate who was going to replace me was going to win the seat in Longford/Westmeath.
"I can also assure the people tonight, we will be going all-out, as and from tomorrow. We will be 150% behind his candidacy."