(Reuters) – Malaysian Prime Minister Najib Razak faces a test of his popularity this week in two by-elections that come hard on the heels of calls by the country’s former long-time leader, Mahathir Mohamad, for him to step down.

The influential Mahathir, 89, has intensified attacks on Najib in recent weeks, criticising him for his management of the economy and scandals arising from debt-laden state fund 1MDB and the murder of a Mongolian model nine years ago.

Mahathir has said the ruling United Malays National Organisation risks losing the next election, due by 2018, if Najib remains its leader. A poor showing in this week’s by-elections could weaken the premier’s position.

… “The results would be used to test whether Najib is a poison pill or a magic wand for his party,” said Oh Ei Sun, a senior fellow at the S. Rajaratnam School of International Studies.

“If the win margin diminishes compared to previous years, then one could argue that Najib’s leadership is dented.”