The 2015 Umno General Assembly this week began even before it formally started. But it was not how Prime Minister and party president Najib Razak would have liked.

His embattled deputy and now nemesis Muhyiddin Yassin stormed the stage on the eve of the gathering with what amounted to his own "pre-assembly curtain-raiser". Complete with the Umno flag and song, and telecast live online, Tan Sri Muhyiddin launched a forceful address in lieu of the traditional platform that was denied him as the elected party No. 2 at the annual assembly that began yesterday.

Speaking as if it was his last stand, Mr Muhyiddin made a pitch to "save Umno", calling on Datuk Seri Najib to step down temporarily to make way for transparent and independent investigations into the twin scandals - the 1MDB issue and the RM2.6 billion (S$850 million) political donations that ended in his personal account.

Mr Najib could resume his post, he said, once cleared of wrongdoing. Criticising Mr Najib for creating a "climate of fear" and becoming increasingly dictatorial by clamping down on those who dared take issue with his leadership, Mr Muhyiddin called for a two-term limit to the tenure of the Umno president.

UNAVOIDABLE OR EXPECTED CURTAIN-RAISER?

Mr Najib's response was swift. In an interview with media chiefs timed for the assembly, he declared his conscience "absolutely clear" as "I have not committed any offence both in law and in manner".

Datuk Seri Ahmad Zahid Hamidi, who took over Mr Muhyiddin's post of deputy premier but not his party deputy presidency, warned Mr Muhyiddin against "crossing the line" and splintering the party. Mr Zahid's reference to splintering could foreshadow the possible sacking of Mr Muhyiddin from his party post. If that comes to pass, Umno is looking at a new split, possibly leading to the exodus of Mr Muhyiddin and his supporters.

Mr Muhyiddin, backed by former prime minister Mahathir Mohamad and other notables, seems resigned to this, while Tun Dr Mahathir said "no risk, no gain".

Mr Muhyiddin had earlier been dropped as deputy prime minister after going public with his concerns over the negative impact of the twin scandals on the party and government. Making clear that he meant no malice, he said it was his duty as a Muslim leader to speak out as he feared Umno could be dragged to court if the current crisis ended up as a legal case against Mr Najib.

Mr Muhyiddin had claimed that despite his pressing for clarification via various internal channels, the response from Mr Najib had neither been sufficient nor satisfactory. Mr Najib preceded his sacking with sweeping claims of a conspiracy to oust him from power, thus making Mr Muhyiddin the third deputy prime minister in three decades to end up in conflict with their boss - after Tun Musa Hitam in the 1980s and Anwar Ibrahim in the 1990s.

The inevitable tensions are now boiling below the surface as delegates gathered to debate Umno's road ahead amid the twin scandals. Forced onto the defensive, Mr Najib briefed the 191 party division heads behind closed doors on the RM2.6 billion political donation and other issues raised by Mr Mahathir.

Mr Muhyiddin's supporters, meanwhile, lodged a complaint to the Registrar of Societies to investigate why the donation was not reflected in the party's account as required by regulations. Mr Mahathir had decided to attend the assembly to see for himself whether the donation would now be reported in the party's financial statement.

TWO POSSIBLE OUTCOMES

This week's Umno gathering could end in two alternative outcomes: If the first - the possible axing of Mr Muhyiddin as party deputy president - comes to pass, it would make his ouster complete. It would end a leadership stint over many decades as the most senior political leader from the highly strategic state of Johor. It is unlikely that Umno Johor has been unaffected by the fate of their leader. Even the Sultan of Johor is said to be concerned and had intimated symbolic support for Mr Muhyiddin after he was sacked - with a photo opportunity at his palace. Mr Muhyddin's choice of venue for his Dec 7 speech was equally symbolic - the Sultan Sulaiman Club in Kuala Lumpur, where Umno was born in 1946.

If Mr Muhyiddin is dropped as party No. 2, he could be thrown into the political wilderness. But if Mr Muhyiddin chose to fight on, he could either initiate, or be forced into, a new political calculus - with or without the opposition. If so, how this may shape up is still unclear, maybe even for Mr Muhyiddin himself. But if Mr Mahathir joins him in the exodus from Umno, the implications on Malaysian politics will be far-reaching, reminiscent of the 1987 split between Team A and Team B, which led to the formation of Semangat 46 led by Tengku Razaleigh Hamzah.

The other possible outcome is even more unpredictable - should traction gather for Mr Muhyiddin's call for Mr Najib to take a leave of absence pending investigations. To be sure, given Mr Najib's strengthening position in the party following a series of manoeuvres for political survival, it is unlikely Mr Najib would step aside at this point.

But it also depends on the party dynamics that Mr Muhyiddin has triggered and which Mr Najib is now trying hard to counter.

TEST OF WILLS AND COMPETING NARRATIVES

Reflecting the growing restiveness, Mr Muhyiddin in his speech claimed that his Facebook account had attracted many likes, including growing calls for Mr Najib to step down completely. While the majority of delegates to the assembly are pro-Najib, Mr Muhyddin is not without his supporters and sympathisers, many of whom have chosen to lie low for now.

By and large, most delegates would prefer this dilemma be resolved without splitting Umno. Youth leader Khairy Jamaluddin said in his opening address that it's "damned if you do, damned if you don't". But there is a "trust deficit" among the people that must be "resolved at all costs".

Much now depends on how the delegates are influenced by the ongoing test of wills between Mr Najib and Mr Muhyiddin and their competing narratives: Mr Muhyiddin, taking the moral high ground, is appealing to the conscience of the party delegates to save Umno by "removing the cancer" that is seriously threatening the party with a defeat at the next general election to be called by 2018. "Will the crisis end if they sack me?" he stated in his Dec 7 pitch.

Mr Najib's counter-narrative is to make the typically Malay appeal to party unity and loyalty to the leadership to solidify Umno ahead of GE2018. Mr Zahid said "one loose rope" will not sink the Umno ship.

Meanwhile, calls are emerging from some delegates for party elections to be speeded up to resolve the leadership crisis. Umno is once again going through an excruciating moment.

How the next general election will be fought and won - or lost - will be defined by this week's crucial event.

•Yang Razali Kassim is Senior Fellow with the S.Rajaratnam School of International Studies (RSIS), Nanyang Technological University.