SYDNEY (Reuters) - It was supposed to relieve a headache for Australian Prime Minister Malcolm Turnbull, but a postal vote on whether to legalize same-sex marriage has awakened interest in politics among young voters who could ultimately turf him out of office.

A pedestrian walks near a poster attached to a light pole supporting a 'Yes' vote in a non-binding poll, conducted by post, to inform the government on whether voters want Australia to become the 25th nation to permit same-sex marriage, in the Sydney suburb of Newtown in Australia, October 4, 2016. REUTERS/David Gray

There had been strong calls from the public for Australia to become the 25th nation to permit gay marriage, a move that Turnbull supports, but more conservative members of his Liberal Party oppose.

Holding a wafer thin parliamentary majority of just one seat, Turnbull opted to call a non-binding ballot as it was politically the least risky way of putting a highly emotive issue on the agenda.

The fierce national debate that ensued persuaded many young Australians to take a far greater interest in politics than they had before.

And while that might help Turnbull make the progressive, liberal amendment to the marriage law that he personally supports, it could cost him in the long term as there is a strong likelihood that the new generation will lean toward the center-left Labor Party rather than his Liberals.

“The issue of same-sex marriage has given the opposition a massive advantage in encouraging more young people, who are far more likely to vote for Labor, onto the electoral roll,” said Simon Banks, one-time chief of staff to former Labor Prime Minister Kevin Rudd.

Young Australians previously have been tardy about registering to vote, knowing that once they do the law requires them to cast their ballot or face a fine of A$110 ($86). But the gay marriage issue persuaded more to get their name on the electoral roll.

“I wasn’t planning to register to vote, I wanted to wait another few years but I wanted to have my say, I wanted to vote ‘Yes’,” said Rebecca Rodgers, 18, a student from Darwin in Australia’s remote north who supports same-sex marriage.

The number of Australians aged under 25-year registered to vote now stands at a record high of nearly 1.65 million, making up just over 10 percent of the electorate.

Looking at Australia’s finely balanced electoral map, there are a number of key marginal seats that could be decided by these young voters the next time the country goes to the polls.

Coming from Solomon, Northern Territories, a constituency where the Liberal Party’s small majority is endangered, Rodgers has already made up her mind.

Slideshow ( 3 images )

“I will never vote for Turnbull,” said Rodgers, an indigenous Australian dissatisfied with the ruling party’s efforts to tackle the social ills afflicting her community.

Labor supporters are hoping that the new generation of voters will deliver a “youthquake” similar to the one that nearly carried Britain’s Labour Party to an unlikely election victory earlier this year.

GOVERNMENT AT RISK

A conservative with social liberal credentials, Turnbull had enjoyed support spread across all age groups since coming to power in 2015.

But that appeal has faded, particularly among young people facing dimmer prospects because of the sluggish economy.

Nearly one-in-five young Australians are underemployed, according to the Brotherhood of St Laurence, an anti-poverty community and advocacy group.

A lack of job opportunities has stoked inequality, and left many young Australians priced out of the property market that has seen home values soar by 69 percent in just nine years.

With opinion polls against him, Turnbull would probably favor hanging on until the last possible moment in 2019 to call an election, but Australia hasn’t been giving its prime ministers the luxury of time.

Turnbull is the country’s fifth prime minister in the last decade, and there is already a constitutional crisis brewing that could precipitate an early election.

Australia’s High Court will rule on Oct. 13 on whether deputy prime minister Barnaby Joyce can remain in parliament after being elected while holding dual citizenship, potentially ruling him ineligible under the country’s constitution.

If Joyce is ruled ineligible and Turnbull is forced to call an election, there will be little scope for him to win over the youth vote without jeopardizing his core support among the more conservative middle-aged and retired voters, political analysts say.

“The Liberal Party has always been very strong among over 55’s so the number one task is for the PM is to make sure that the core, base vote is as strong as he can make it,” said Grahame Morris, former chief of staff to John Howard, Australia’s second longest serving prime minister.

“If that base walks away, nothing else matters.”