Sri Lankans elect a new parliament on Monday in what amounts to a referendum on ex-president Mahinda Rajapaksa's comeback bid, with the reformist alliance that swept him from power in January seeking a stronger mandate for reforms.

The nationalist strongman has set his sights on becoming premier of a government led by his Sri Lanka Freedom Party (SLFP). But the former ally who beat him at the polls in January, President Maithripala Sirisena, now leads the party and he rules that out.

The tangled personal rivalry has overshadowed campaigning on the Indian Ocean island of 20 million people, which has a history of political feuding that has often spilled over into violence and even the assassination of its leaders.

Sirisena, in a cross-party alliance with a government led by the United National Party (UNP), has sought to break with that troubled past by passing reforms to weaken his own presidency and make the government more open and accountable.

"We all united and voted against Rajapaksa to send him into retirement," Prime Minister Ranil Wickremesinghe said at his last campaign stop on Friday night. "If he hasn't got the message, we should unite to make sure he understands it now."

Minority Tamils and Muslims have rallied behind the centre-right coalition led by Wickremesinghe's UNP, which pundits say has the best chance of forming the largest bloc in the 225-seat parliament.

A stronger UNP mandate would help complete the "unfinished business" of the reform process that has stalled because the party and its allies now lack a majority, said political analyst Paikiasothy Saravanamuttu.

Rajapaksa, 69, is revered as a war hero by many of Sri Lanka's Sinhala speaking Buddhist majority for crushing a 26-year Tamil uprising in 2009. Opponents accuse him of running a corrupt, brutal and dynastic regime - charges he denies.

"This government chased the investors away and welcomed the underworld. In our period, we chased the underworld and brought investors," Rajapaksa has said in answer to his critics.

At stake for the wider world is whether Sri Lanka sticks to its pro-Western course or turns back towards China. Under Rajapaksa, Beijing pumped billions of dollars into making the island part of a new "Maritime Silk Route".

IT'S PERSONAL

Sirisena quit Rajapaksa's government last year to run against him, pulling off a stunning victory in presidential elections on Jan. 8.

Yet he has moved only belatedly to assert his control over the SLFP and to block the path to the premiership of his erstwhile ally and party rival.

In a widely leaked letter, he accused Rajapaksa of holding the party "hostage" and ruled out naming him prime minister. Sirisena, 63, has also used his power as party leader to purge Rajapaksa loyalists from key posts in recent days.

The manoeuvring could determine whether Sirisena can form a unity government comprising Wickremesinghe's centre-right alliance and his own loyalists from the SLFP, sending Rajapaksa to the opposition benches, say analysts.

In the election, 196 lawmakers will be elected from party lists in multi-member districts. The rest will be elected from national lists, with party leaders deciding who gets a ticket.

Fifteen million people will have the right to vote after what observers say has been an unusually clean campaign.

Polls open at 7 am (0130 GMT) and close at 4 pm. The final count is due on Tuesday.