By Padraic Halpin

DUBLIN (Reuters) - Four lawmakers from Irish Prime Minister Enda Kenny's party came out on Tuesday against a move to legalize abortion under certain conditions but are unlikely to be able to scuttle the measure or threaten his comfortable majority.

Kenny's government is proposing access to abortion when a woman's life is in danger and both sides of the debate have staged protests on an issue that has long polarized the overwhelmingly Roman Catholic country.

A two-decade debate over how Ireland should deal with a Supreme Court ruling that abortion be permitted when a woman's life is in danger was reopened last year after the death of a woman who was denied an abortion of her dying fetus.

Successive governments had sidestepped acting on the ruling, the result of a challenge by a 14-year-old rape victim in the so-called "X-case" of 1992 to a constitutional amendment nine years earlier that intended to ban abortion in all instances.

Midway through his five-year term, Kenny had kept all but one of Fine Gael's 76 members of parliament on side, even as the coalition government pushed through tough austerity measures required under an EU/IMF bailout.

The government has the largest majority on record and support from most of the opposition, meaning 138 of the lower house's 165 members supported the abortion bill. It will become law after a final vote is held, probably before the end of July.

But at least five other members of Kenny's center-right Fine Gael, including European Affairs Minister Lucinda Creighton, have said they hope to convince fellow lawmakers to amend the bill before then, raising the prospect of more defections.

Kenny, who has said he has been sent letters written in blood by opponents of his plans, has ruled out making any substantive changes including removing the threat of suicide as a reason for termination, a contentious issue for his party.

He insisted the government voted as one on the bill, meaning Tuesday's four backbench rebels face expulsion from the party.

(Reporting by Padraic Halpin; Editing by Mark Heinrich)