By Karolina Slowikowska and Christian Lowe

WARSAW (Reuters) - Polish Prime Minister Donald Tusk and the leftist opposition could form a coalition after the next election, senior figures from both parties say, a deal that would keep Tusk in power and commit Poland to a timetable for joining the euro.

Tie-ups between the parties - or their predecessors in Poland's fractured political landscape - have never got off the ground in the past because the ideological divide was too great. This time people on both sides say the gap is narrowing.

After six years in office, Tusk's party is trailing in the opinion polls behind the conservative Law and Justice party, and has been damaged by a brief slowdown in economic growth and a sense among some voters his administration has lost its way.

The opinion polls suggest Law and Justice will not have enough seats in parliament to form a government on its own, and the other major parties say they will not contemplate entering into a coalition with it.

In this situation, the leftist SLD party could be a lifeline for Tusk, joining him in a coalition that would have a majority in parliament and allowing his center-right Civic Platform party, known by its Polish acronym PO, to stay in power.

But that support would come at a cost: the SLD, led by reformed ex-Communists, would only join a coalition with Tusk if he waters down some of his liberal economic policies and also commits to a road map for joining the single currency.

There are still many unknowns about the shape of the next government. Nevertheless, the prospect of an SLD-PO alliance is likely on balance to reassure markets, who are nervous about the prospect of Tusk ceding power to the conservative opposition.

"There is little probability that PO could win outright and so it will have to look for a coalition partner. That's where SLD will come in. In fact, there won't be much choice," said Iwona Jakubowska-Branicka, a professor at Warsaw University.

Senior figures in both parties say the PO is less right-wing after the defection of leading figures in its conservative faction last month. Out of power for almost a decade, the SLD has also shaken off more of the baggage from its origins in the Polish Communist party.

In an interview with Reuters last month, SLD leader Leszek Miller said he would consider forming a coalition government with the Platform, provided the two sides could agree to a binding agreement on policies they would jointly pursue.

A senior lawmaker with Civic Platform, who spoke on condition of anonymity, said that the party was looking, among other options, at the possibility of a coalition with the SLD, especially if the Platform came second in the next election.

The PO lawmaker said a decision by the SLD to back the government on a fiercely-contested overhaul of the pension system showed the party was becoming more constructive.

COMMON GROUND

Tusk has said the next election could happen earlier than scheduled if his government loses its majority in parliament, now wafter-thin after several lawmakers defected.

In the interview, Miller said hammering out a coalition with Turk's party would not be an easy task. But he said: "The easiest part would be for us to reach agreement on issues of European integration."

He said it was a pity Tusk had not set a timetable for euro entry. Poland was ready to join, he said, though probably would have to wait until after 2019 for political reasons.

"We should have a road map for euro entry," said Miller, who as Polish prime minister from 2001 to 2004, guided Poland into the European Union.

"If we are not in the euro zone, we will be second-category members. So we cannot afford or accept this if we want to be part of the core (of European nations)."

Until the euro zone debt crisis set in 2009, Polish governments - including Miller's 2001-2004 administration - had continuously set target dates for joining the euro only to push them back by years. Tusk has declined to give a new date.

On economic policy, Miller, once a senior official in the Communist Party that ruled Poland for four decades, said his party differed from Tusk on how much help to give the poor and vulnerable.

But he said: "We could also agree on the main economic policy issues, such as the idea that market economics are the most effective."

Most opinion polls show Law and Justice slightly ahead of the Civic Platform, with the SLD in third place and the Peasants' Party, which is Tusk's coalition partner now, hovering around the 5 percent threshold needed to get into parliament.

Many analysts say Law and Justice, despite its popularity with conservative-minded voters, is unlikely to form a government because the polarizing style of its leader, Jaroslaw Kaczynski, pushes away potential coalition partners.

His term as prime minister in 2006 and 2007 was marked by damaging rows with neighbors Germany and Russia that left Poland isolated from the European mainstream.

(Editing by Patrick Graham)