A campaign by young Malaysians to cast protest votes is picking up steam as the Southeast Asian country braces for a potentially acrimonious national election between two candidates that must be held by August.

Prime Minister Najib Abdul Razak faces a challenge from his former mentor and premier Dr Mahathir Mohamad, who turned on him over a multi-billion dollar scandal involving a state fund and now leads the opposition campaign to oust Najib's coalition. Najib denies wrongdoing.

But youths pushing the #UndiRosak, or spoil your vote, campaign on social media, say they refuse to pick between Mahathir, 92, and Najib, 64, whom they blame for building the current political system.

The plan - cast your votes, but mark your ballots wrongly to make sure no political party benefits.

"There is a democratic deficit. We want to leave the BN-PH binary because honestly, it's getting tiring," social activist Maryam Lee said, using the acronyms for the ruling Barisan Nasional and opposition Pakatan Harapan pacts.

Support for a boycott is strongest among the young voters who the opposition need to challenge Najib. The ruling coalition is widely expected to win the polls.

Mahathir has tried to reinvent his image on social media, but his reputation as an iron-fisted dictator over 22 years in charge continues to divide opinion.

"The opposition are not entitled to our votes just because BN sucks. You have to work for my vote," Lee, 25, said on Thursday at a forum to debate the merits of the movement.

The #UndiRosak campaign gained momentum in the weeks after the opposition named Mahathir their candidate for prime minister earlier this month.

More than 1,500 Twitter users have discussed the topic over the past two weeks, according to a survey by social media research firm Politweet.

#UndiRosak is also the second-highest trending Twitter topic in Malaysia.

While the campaign is small - Lee thinks it will only influence one percent of the 14.6 million registered voters – it has jolted the opposition.

Mahathir said this week that it was "shallow-minded" for the youth to not cast their vote.

"We need to change this country. If we didn't need to, I wouldn't bother getting into this because I'm 92, and I'm going to turn 93," he said at a media briefing on Tuesday.

Is your 'powerful' is your vote? Visit Undi Power to find out.

In the 2013 national polls, 2.6 million new voters helped push a record turnout of over 84 percent and lose Najib's ruling coalition the popular vote.

But between 2013 and 2017, the number of new voters went down to two million, according to estimates by independent polling firm Merdeka Center.

Another two million have yet to register. Umno said the boycott only proved that Mahathir and the opposition were not popular.

"Those who will not come are those supporting the opposition but disillusioned with the opposition," Abdul Rahman Dahlan, a member of Umno's powerful supreme council, was quoted as saying on Thursday by state newswire Bernama.

- Reuters