Two by-elections have been set for June 18, and both will serve as a key test for the ruling Barisan Nasional (BN) and Prime Minister Najib Razak, as well as the opposition.

The two polls will be the first to take place on the peninsula since a scandal erupted over alleged financial mismanagement at the state investment firm 1Malaysia Development Berhad (1MDB) and Mr Najib's role in it.

The stakes will also be high for the opposition because of their disastrous performance in the Sarawak state election last week which saw BN, led by Chief Minister Adenan Satem, winning 72 of the 82 seats at stake.

Yesterday, Tan Sri Adenan named a nine-member Cabinet, including three deputy chief ministers. All were sworn in before Sarawak Governor Tun Abdul Taib Mahmud.

Analysts attributed BN's huge victory in the east Malaysian state mostly to the fact that local development issues were foremost in voters' minds. Umno has no presence in the state and the 1MDB saga did not factor in the ballot.

The two by-elections - in Kuala Kangsar in Perak and Sungai Besar in Selangor - were triggered after their MPs were killed in a helicopter crash while campaigning on behalf of BN candidates in Sarawak.

Umno Supreme Council member and Tourism Minister Nazri Aziz told reporters yesterday: "Whatever the results, they will have an impact on the Prime Minister. So we must be careful."

The opposition will also need to come to a compromise as Parti Islam SeMalaysia (PAS) and its splinter party, Amanah, are both staking their claims on the Muslim-majority seats.

Nominations for the by-elections will be held on June 5, just a day after PAS' annual national congress ends.

PAS was once part of the Pakatan Rakyat opposition coalition which is now in tatters after PAS split with its partners, Parti Keadilan Rakyat and Democratic Action Party, over implementing Islamic criminal law and the choice of a chief minister in Selangor.

A group of disaffected PAS leaders has formed Amanah and joined the new opposition alliance, Pakatan Harapan. But ties in the new alliance have also proven shaky, culminating in overlapping claims and a disjointed campaign in Sarawak that saw the opposition losing five seats after a decade of gains in the state assembly.

Policy think-tank Institute for Democracy and Economic Affairs chief executive Wan Saiful Wan Jan told The Straits Times: "It's not just about Najib's popularity.

"Voters are equally angry with the opposition's failure to work together as well."