Malaysian Prime Minister Najib Razak may have secured a resounding win for his coalition in Sarawak's polls last weekend, but he faces a fresh challenge in the peninsula where two by-elections will soon be held, after a helicopter crash in Borneo killed two Umno MPs.

"This will be an actual test of Najib's popularity," said Mr Wan Saiful Wan Jan, political analyst and chief executive of think-tank Institute for Democracy and Economic Affairs (Ideas).

Datuk Seri Najib led the Sarawak election campaign and committed tremendous resources by having Cabinet ministers go on the stump in the state. He also announced millions of dollars' worth of infrastructure development. But analysts say it is a different ball game for Mr Najib in the peninsula, where voters are tired of rising costs of living and a leader plagued by financial scandals.

"It's a litmus test of Umno's ability to continue to gain support in West Malaysia," said Mr Ibrahim Suffian, director of independent pollster Merdeka Centre.

The by-elections will be held in Sungai Besar in Selangor and Kuala Kangsar in Perak. In the 2013 general election, the Umno MPs had won the two racially mixed constituencies by a narrow margin.

Sungai Besar had 18,696 voters, of whom about 66 per cent were Malays, about 31 per cent Chinese and about 2 per cent Indians in 2013.

Kuala Kangsar then had 14,218 voters comprising about 68 per cent Malays, about 24 per cent Chinese and about 7 per cent Indians.

The seats were held by the late Datuk Noriah Kasnon in Sungai Besar and Datuk Wan Mohammad Khairil Anuar Wan Ahmad in Kuala Kangsar, both from the United Malays National Organisation (Umno).

They were among six people who died in a helicopter crash on May 5, at the tail end of campaigning for the Sarawak polls.

Traditionally, the contest in the two seats has been between an Umno and a Parti Islam SeMalaysia (PAS) candidate. Islamist party PAS has been on friendlier terms with Umno after falling out of the opposition alliance that was Pakatan Rakyat.

"This by-election is going to be an important indicator, not just of the electability of Umno but the nature of PAS' new relationship with Umno," said Mr Ibrahim.

In the 2013 general election, Madam Noriah won Sungai Besar by 399 votes. In Kuala Kangsar, Mr Wan Mohammad won by 1,082 votes.

With so few votes needed to tip the scales against Umno and plenty of room for surprises, Mr Najib may not be able to rest on his laurels yet after the thumping victory in Sarawak. "It is not just about the level of dissatisfaction shown by normal voters, but also a test of what Umno members think about Najib," said Mr Wan Saiful.

Losing two seats to the opposition would not topple the BN government in Parliament, a fact that voters are aware of. For Umno supporters in the semi-urban and rural areas which make up the two constituencies, voting against the party in the by-elections would be a form of showing displeasure with Umno leaders.

"This is the lowest-risk method to protest," said Mr Wan Saiful of Ideas, referring to the anonymity of voting as the safest way for unhappy Umno members, who could otherwise be disciplined by the party for not toeing the line.

On the other hand, if Umno retains the two seats, Mr Najib could claim that voters remained firmly behind him despite accusations of financial irregularities.

•Additional reporting by Shannon Teoh