Malaysia's opposition alliance yesterday named former premier Mahathir Mohamad as its prime ministerial candidate for a general election this year, although the authoritarian who ran the country for two decades and is now 92 years old faces an uphill task.

With the country's most popular opposition leader Anwar Ibrahim in jail, Mahathir is seen as the biggest threat for premier Najib Razak, who must call an election by August but is engulfed in a corruption scandal.

Still, an independent survey showed the opposition would have difficulty beating Najib, due to divisions within its own camp and unfavourable electoral boundary changes.

Mahathir and former foe Anwar have joined up to oust Najib, and an election victory by their alliance could also potentially pave the way for Anwar to return and take over as prime minister.

If the opposition wins, it would immediately start the process to obtain a royal pardon for Anwar so that he can be eligible to become prime minister, secretary general Saifuddin Abdullah said at the alliance's convention.

Anwar's wife Wan Azizah Wan Ismail will be the coalition's candidate for deputy prime minister.

Mahathir, who during his 22-year tenure as prime minister earned a reputation of being a no-nonsense authoritarian with little time for dissenters promoting liberal values, stands to become the world's oldest leader if the opposition wins.

The Mahathir-led opposition, however, looks unlikely to unseat the ruling Barisan Nasional (BN) coalition, according to a survey conducted by independent polling firm Merdeka Center in December.

The survey showed that although BN's share of the popular vote will shrink further, it can potentially regain a two-thirds majority in parliament, The Malaysian Insight reported yesterday.

Merdeka Centre's director Ibrahim Suffian said the Mahathir-led group was at a disadvantage due to a split in the opposition and the redrawing of electoral boundaries that critics say favour the ruling government.

The Pan-Malaysian Islamic Party (PAS), which was part of the opposition group in 2013, is no longer part of the pact.

"We are highly sceptical of the opposition's prospects if they continue to be split as things are now," Ibrahim said.

Malaysia's political landscape has been shaped for nearly two decades by a bitter feud between Anwar and Mahathir.