DUBLIN (Reuters) - Support for left-wing Irish nationalists Sinn Fein surged in an election on Saturday, leaving it tied with the party of Prime Minister Leo Varadkar but unlikely to emerge with the highest number of seats, an exit poll showed.

Instead Varadkar’s Fine Gael is likely to compete with fellow centre-right rival Fianna Fail to secure the most seats and the right to try to form a coalition government - a task analysts called extremely difficult.

The Ipsos MRBI exit poll showed Fine Gael on 22.4%, Sinn Fein on 22.3% and pre-election favourites Fianna Fail on 22.2%.

Sinn Fein, the former political wing of the Irish Republican Army (IRA), is likely to fall behind the other two parties because it fielded far fewer candidates.

Still, its breakthrough represents a major realignment of Irish politics, which for a century has been dominated by Fine Gael and Fianna Fail.

“It’s a very, very good result for Sinn Fein... but even though it is a statistical tie, we would expect Fine Gael and Fianna Fail to fight it out to get the most seats,” said Gary Murphy, Professor of Politics at Dublin City University.

Fianna Fail has ruled out going into coalition for the first time with Fine Gael and both parties say they will not govern with Sinn Fein, meaning there is no obvious government to be formed, he added.

“There are very rocky times ahead,” he said.

“HISTORICAL ELECTION”

Counting begins at 0900 GMT on Sunday with some results expected from early afternoon. The final and potentially decisive seats in Ireland’s 160-member parliament may not be filled until Monday or even later.

Fine Gael and Fianna Fail have said they will look to smaller parties to form what would likely be another minority government requiring support of one of the two main parties from the opposition benches.

The parties have swapped power at every election since emerging from the opposing sides of Ireland’s 1920s civil war. They have similar policies on the economy and Brexit trade talks.

Sinn Fein has moved on from the long leadership of Gerry Adams, seen by many as the face of a bloody IRA war against British rule in Northern Ireland. Its candidates were the biggest gainers by vote share, up from 14% at the last election in 2016.

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With Fine Gael and Fianna Fail marginally down, the outcome demonstrated some appetite for change.

“The days of Fine Gael and Fianna Fail dominating Irish politics are over,” Fine Gael parliamentary party chairman Martin Heydon told national broadcaster RTE.

“Our elections are becoming more volatile. More like a lot of other European countries. The ability to form governments is going to be hard after this.”

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A new generation of politicians led by Mary Lou McDonald spearheaded the groundswell of support for Sinn Fein, particularly among younger voters on the defining election issue of the cost and availability of housing.

Her deputy leader in the Irish parliament, Pearse Doherty, said the exit poll represented a vote of “a historic nature” for the party.

“You can see in the figures there today that there is a mood for change,” he told RTE.