A president who has to consult his council of advisers before exercising his veto on critical matters will be hamstrung in his role as a custodian, Workers' Party (WP) MP Png Eng Huat (Hougang) said.

Arguing against the changes to the elected presidency, he said the president would have to "share power" with an enlarged Council of Presidential Advisers (CPA) in the discharge of his custodial roles.

The changes require the president to consult the CPA on all fiscal matters relating to the reserves and key public service appointments. There will also be eight CPA members, up from the current six.

"The CPA, though only an appointed body, is bestowed with considerable powers to challenge and delay the decision of the president," said Mr Png, adding that a presidential veto does not carry weight unless it had the support of the CPA.

He also argued that the CPA, in its "enlarged" role, now functions like an "Upper House" of the legislature.

"The CPA has the power to turn the elected president into a lame duck institution over time if it chooses to disagree with the president each time he uses his veto. Check and balance under a political framework must be legitimised and mandated by the people," he said.

Replying, Deputy Prime Minister Teo Chee Hean said WP "fundamentally misunderstood" the CPA's role, and was misleading the House.

"It is not a third centre of power. It is unable to single-handedly block a government proposal. The CPA's role has been the same for the past 25 years, and remains unchanged under the Bill," he said.

The changes "merely adopt a uniform approach to the president's obligation to consult the CPA on financial and appointment-related matters, as well as extend the overruling mechanism to these areas".

The WP had on Tuesday called for a return to an appointed presidency, with the custodial functions of the president vested instead in an elected senate.

The party also asked for a referendum so people could vote for the system they wanted.

Yesterday, WP Non-Constituency MP Leon Perera also questioned why an unelected CPA was vested with so much power. "This creates a president supposed to be a checker who is prone to be checkmated by an unelected council," he said.

DPM Teo said in situations where the president has vetoed a proposal the CPA agrees with, its views only serve to "moderate the weight" of the president's veto.

Parliament would have to relook the proposal, which can only be passed with a two-thirds majority.

"Ultimately... it is always an elected institution - the Parliament or the elected president that makes the decision," said DPM Teo.