DUBLIN — Ireland’s presidential election will feature only three candidates — the fewest in 35 years.

Catholic conservatives on Wednesday decried the narrow failure of their candidate, anti-abortion campaigner Maria Steen, to secure a spot on the Oct. 24 ballot to become Ireland’s next head of state.

Steen needed official endorsements from at least 20 lawmakers to be listed, but she fell two short of the constitutional requirement. She and her supporters predicted that hundreds of thousands of right-wing voters would spoil their ballots or boycott the election in protest.

The race now will pit one anti-establishment lawmaker from Ireland’s left-wing opposition, Catherine Connolly, against politicians from the two center-ground parties in Ireland’s coalition government: Cabinet veteran Heather Humphreys of Fine Gael and sports hero Jim Gavin of Fianna Fáil.

This marks the fewest candidates to qualify for a presidential ballot since Ireland’s watershed 1990 election, when Mary Robinson, a civil rights lawyer from the opposition Labour Party, became the first non-Fianna Fáil figure to win the presidency in a three-way contest. Connolly, an independent socialist with backing from both Labour and the main opposition Sinn Féin, will be hoping history repeats itself.

Fine Gael, Ireland’s most pro-EU party, has never won a presidential election. But this time, all opinion polls have placed Humphreys in pole position, reflecting her reputation as a popular figure and canny campaigner in rural Ireland. That early consensus is backed by gambling markets, which rate the Fine Gaeler as the safest bet.

Ireland’s president has no role in government but must sign all legislation before it can become law. This review power means that, in relatively rare cases, the president can refer legislation already approved by the parliament and government to the Supreme Court to determine whether it is constitutional.