A campaign to end Ireland’s constitutional ban on abortion appears to have moved one step closer to success.

In an emotional speech to Parliament late Thursday, Micheal Martin, the leader of the conservative Fianna Fail party, said that he would support a forthcoming referendum to repeal the constitution’s Eighth Amendment, which has been interpreted as prohibiting abortion in almost all cases, including rape, incest and fatal fetal abnormality.

Mr. Martin’s surprise intervention appeared to steal political momentum from his rival, Prime Minister Leo Varadkar, who leads the Fine Gael party, which is also conservative but is generally more liberal on social issues. While Mr. Varadkar’s government is committed to holding a referendum this year, he has so far declined to state his own position on abortion.

Speaking in the western city of Limerick on Friday, Mr. Varadkar told journalists that he had previously said that he believed the present law was too strict and should be eased. But he also said that he wanted to listen to all shades of public opinion before deciding, with his cabinet, how the law should be changed. He said that he did not feel pressured by Mr. Martin’s surprise declaration.