PKR vice-president Xavier Jayakumar said MPs who have quit Umno and are looking to join Pakatan Harapan should also show their commitment to prime minister-in-waiting Anwar Ibrahim.
Xavier said this was among the conditions laid out by the Harapan presidential council and its respective leaders.
"In addition to recognising and supporting Mahathir as the present prime minister, they should reaffirm the same unwavering commitment for the prime minister-in-waiting," he said in a statement today.
Most importantly, Xavier noted, MPs should also take the opportunity to reveal and whistleblow on any other allegations of corruption involving current Umno leaders, such as former president Najib Abdul Razak and his successor Ahmad Zahid Hamidi.
He added that they should also commit themselves to Harapan's cause of bringing positive change for the country by supporting the relevant legislation in the Dewan Rakyat.
"Major changes by Harapan which are deemed positive for the rakyat and may require a two-thirds majority should also be supported by these said MPs."
Xavier said as long as the MPs fulfil these conditions, they may join Harapan, but may also remain as independents if they wish.
"Jumping and supporting the present ruling coalition instead of building up a credible opposition pact should come with a price.
"The price of committing to the Harapan cause as voted by the majority of the rakyat must be one of absolute commitment and not one of 'oppurtunitism' for the sake of 'cosying' with the winning side," he said.
Umno was the largest party with 54 MPs despite losing power in the 14th general election.
It is now left with 38 elected representatives in the Dewan Rakyat, making it the third largest party after PKR and DAP.
This is after another 10 MPs quit Umno since Thursday, accusing Umno president Ahmad Zahid Hamidi of failing to chart a direction for the party.