Shadow Agriculture Secretary Tim Yeo, a supporter of Mr Portillo, said the party needed to engage with the issues that mattered to ordinary people or it would not deserve to be re-elected.
A possible rival, shadow home secretary Ann Widdecombe, said she would not serve in a Portillo shadow cabinet unless his supporters stopped "backbiting".
Miss Widdecombe - along with Iain Duncan Smith, and Kenneth Clarke - are still considering their positions.
Meanwhile the beleaguered party received a boost on Tuesday evening when it emerged that philanthropist Sir John Paul Getty II had donated £5m to its coffers.
He said he hoped the money would help a new leader win the next election.
Speaking on BBC Radio 4's Today programme, Miss Widdecombe said she thought it unlikely she would be invited to serve in the shadow cabinet if Mr Portillo became leader.
Backbiters
Asked if she would accept the offer if asked, she said: "I think the difficulty there would not be Mr Portillo because once someone has been elected it's the duty of people to get on and serve.
"The problem would be the little band of backbiters who surround him.
"I don't think I could go through another four years of that."
Pressed on whether she would accept a place in a Portillo shadow cabinet under any circumstances, she said: "Unless there is a revolution in the way he controls his supporters the answer's 'no'."
Miss Widdecombe said she would not follow Mr Portillo by making a declaring her candidature on Wednesday and declined to define the ideological differences between herself and the shadow chancellor.
She said letters from grass roots supporters had been "flowing in" but she needed to gauge her support among her fellow MPs.
Shadow Agriculture Secretary Tim Yeo, a supporter of Mr Portillo, said the party needed to engage with the issues that mattered to ordinary people or it would not deserve to be re-elected.
"I think we are at a defining moment in the Conservative Party's history when it's not just a question of whether we can win the next election but the actual survival of the party as an important political force," he said.
Low-key statement
Mr Portillo is currently the bookies' favourite to become the next Tory leader.
Although he is drawing support from MPs on the centre and left, some former allies on the Tory right have been critical of his attempts to broaden his appeal.
Supporters of the former chancellor Kenneth Clarke say the lines of communication between him and Mr Portillo remain open.
Party rules mean Tory MPs select two candidates to go forward to a vote of all party members.
John Strafford, chairman of the Campaign for Conservative Democracy, criticised that system on Wednesday.
He told BBC News: "It is not democratic for the Conservative parliamentary party to say to the members, you can vote in the leadership election but we will tell you the candidates and we well restrict the candidates to two."
The expected start of the Tory leadership battle - which follows the resignation of William Hague - will coincide with the arrival at parliament of MPs still flush from electoral success last Thursday.
New arrivals
The first business of the Commons will be the election of the Speaker Michael Martin, who held the job until the last parliament was dissolved.
The new Leader of the Commons, Robin Cook, will address the first meeting of the new Parliamentary Labour Party.
Among the new arrivals will be the Independent MP Dr Richard Taylor who defeated junior government minister David Lock in the Worcestershire seat of Wyre Forest.
Dr Taylor ran on the platform of saving services at Kidderminster general hospital.