Sinn Féin's Pearse Doherty has moved up to 15,188 votes after the third count in the Donegal South West by-election and is in poll position to take the seat where the quota is 17,213.
Independent candidate Thomas Pringle has been eliminated on the third count after only getting 3,763 votes.
Fianna Fáil's Brian Ó Domhnaill has 7,636 votes and Fine Gael's Barry O'Neill has 7,313.
Labour's Frank McBrearty was eliminated on the second count.
Mr Doherty looks set to take the seat, which was vacated by Pat "The Cope" Gallagher 18 months ago.
It is another blow for Brian Cowen, whose Government is preparing to deliver a hair shirt budget in under two weeks.
The results of the third count are:
Pearse Doherty (SF) +1452 = 15,188
Brian Ó'Domhall (FF) +278 = 7,636
Barry O'Neill (FG) +871 = 7,313
Thomas Pringle (Ind) +272 = 3,763
Non Transferrable +502 = 524
Thomas Pringle Eliminated and fourth count proceeds