That prospect has prompted the left-leaning Labour party, which had previously ruled out entering a coalition with Ahern, raising the possibility of a deal with the prime minister's Fianna Fail party.
Pat Rabbitte said the prospect of sharing power with Ahern did not really appeal to him but he did not "want to see Sinn Fein driving economic policies or other policies".
Ahern has ruled out governing with Sinn Fein, saying the party's left-wing economics are incompatible with his own policies, but opinion polls show voters believe he would do a deal if it was the only way to extend his 10 years in power.
Ahern is one of Europe's longest serving leaderships and under his stewardship the economy has grown considerably and peace has come to Northern Ireland.
But allegations over payments from friends and businessmen in the 1990s when he was finance minister undermined his campaign for a third term early on.
The governing coalition of the centrist Fianna Fail and the pro-business Progressive Democrats will fall short of a majority.
Fine Gael, the main opposition party, is fighting to unseat Ahern on a joint platform with Labour but they too could fail to win enough seats even if they do get the backing of the so-far unaligned Green Party in a 'rainbow coalition'.
"Voting Labour would normally be the last thing I would do," Ruth Jenkinson, 31, said as she cast her vote in south Dublin.
"But arrogance and complacency have been creeping in to ministerial posts and power does breed corruption, especially if a party has been in power for too long."
The opposition has also tapped into a sense that the wealth generated by Ireland's "Celtic Tiger" boom is being squandered.
Fine Gael and Labour say they will do better at running a "shambolic" health service and have promised to bring Ireland's transport network up to speed.
Ahern has won back some lost ground in recent polls after publishing receipts to try to dispel doubts over his finances and following an assured television performance.
"There's no doubt Bertie will win," said retired plasterer Alex McQuillan after voting in Ahern's Dublin constituency. "There's no one else there, no alternative leader."
Polls close at 10:30pm (0930 GMT) with counting due to begin on Friday morning. A close result may lead to days of horse-trading as parties try to cobble together a majority.