Pakatan Harapan and PAS' inability to reconcile on a common candidate for the opposition in the Kuala Kangsar and Sungai Besar by-elections may see Umno retaining both seats in three-corner fights.

However, political analyst Wong Chin Huat said the key determinant in both by-elections will be the ability of former premier Dr Mahathir Mohmad and his rebels to hurt Umno.

"The infighting doesn't help the opposition in the two by-elections, it might lower turnout, as in the Sarawak election.

"However, the key determinant on the by-elections outcome is not the opposition's unity, but the resolve of Mahathir and Umno rebels to break with Umno," he told Malaysiakini.

He pointed out that even though the Pakatan Harapan and PAS are insisting that they be allowed to take on Umno in a direct contest, this does not necessarily guarantee a win for the opposition.

Wong (photo), who is a Penang Institute fellow, pointed out that with the disintegration of Pakatan Rakyat and PAS' departure, not all opposition supporters will be willing to support PAS or Pakatan Harapan even if they were the sole candidate from the opposition.

"While Pakatan won 47 to 49 percent of the votes in 2013, some of their votes will now only vote for PAS and not Amanah or PKR while others will only vote for Amanah or PKR but not PAS even if there was only one opposition candidate.

"For Najib to suffer a humiliating defeat, there must be a split of Umno's support base," he said.

Umno rebels playing safe

However, Wong questioned whether the Umno rebels such as Mukhriz Mahathir and Muhyiddin Yassin would be willing to go all out against their own party which would completely sever their ties to the party.

"So far, with the exception of Mahathir, the Umno rebels are playing safe to avoid being sacked.

"And Najib too wants to play safe by not sacking them," he said.

Wong said if this scenario persists, then Umno would stand a good chance of defending both marginal seats.

"If both sides continue to play safe, Umno will retain the by-elections.

After all, if Mukhriz (is unwilling to make a move if) has too many things to lose, why should Umno grassroots sacrifice themselves for his victory in the power struggle?" he said.

As such, Wong said the by-elections will be a litmus test for both sides.

"The by-elections are barometers not only for Najib's popularity but also Mahathir's influence," he said.

Umno won the Sungai Besar seat by a 399-vote majority and the Kuala Kangsar seat by a 1,082 majority in the 2013 general election.

The seats fell vacant after the incumbents Noriah Kasnon and Wan Mohammad Khair-il Anuar Wan Ahmad were killed in a helicopter crash on May 5.

The by-elections are set to take place on June 18.