Prime Minister Bertie Ahern pushed for the amendment excluding suicide threats as legal grounds for abortion. Ireland's constitution requires public approval for any changes to its stern anti-abortion law - the toughest in Europe.
But the results from Wednesday's referendum, announced Thursday, demonstrated a profound lack of consensus on what remains the most divisive social issue in this nation.
Those voting against the proposed amendment prevailed by just 10,556 votes out of 1.25 million cast - a difference of less than 1 percent. Political analysts said the referendum probably was defeated by a split within Ireland's many anti-abortion groups, who campaigned on both sides of the issue.
The vote also was a defeat for the Roman Catholic Church since bishops each issued letters calling for churchgoers to support the amendment. The bishops had no comment on the result.
The proposed amendment was intended to solve the contradictions in interpreting the 1983 constitutional amendment banning abortion, which also recognized the equal right to life of the mother and the fetus.
A 1992 Supreme Court judgment ruled that the constitution's protection of the life of the mother included threats of suicide.
The ruling involved the case of a 14-year-old Dublin girl, identified only as X, who was raped and impregnated by a family friend and then prevented from traveling to England for an abortion.
In the decade since, abortion in Ireland has inhabited a legal netherworld, with doctors performing an unknown number of life-saving terminations but none on women diagnosed as suicidal.
Doctors have been deterred partly by the threat of lawsuits from anti-abortion activists, a risk made greater because successive governments have refused to pass legislation supporting the court's 1992 verdict.
Ahern wanted to overrule the Supreme Court view on suicide because, he and Catholic Church leaders argued, women would make false suicide threats to obtain legal abortions.
In the wake of the defeat, Ahern and his most loyal lieutenants were at pains to claim it would not harm his Fianna Fail party's prospects in the general election expected in May or June.
But the setback meant Ahern went into Friday night's start to the annual conference of Fianna Fail, Ireland's perennially biggest party, sporting a record of consistent failure at the polls since rising to power in 1997.
Under his watch, the party has lost five parliamentary by-elections and, in the past year, two referendums. Voters last year embarrassed Ahern by rejecting the European Union's latest treaty setting the stage for admitting eastern European nations.
Opinion polls consistently rate Ahern as Ireland's favorite party leader, largely because of his easygoing manner and common touch lacking in his rivals. But his latest miscalculation, opposition chiefs argue, illustrates chronic incompetence in his administration that voters simply cannot reward.
Michael Noonan, leader of the main opposition Fine Gael party which campaigned against the amendment, said Ahern had launched ``a personal crusade and then he turned his back on the debate. When the heat came on, he wanted to be on both sides of it as he always does.''
Ahern was criticized for his approach to the campaign, in particular for refusing a television debate with Noonan, who has promised to pass legislation in line with the Supreme Court verdict if he becomes prime minister.
Ahern was noncommittal on that issue.
All parties do support one key next step in Ireland's anti-abortion policies - creating a Crisis Pregnancy Agency to counsel many of the approximately 7,000 Irish women who travel to England each year for abortions.