Mr Kenneth Clarke yesterday admitted he wanted to be British Conservative Party leader but insisted he was undecided on whether to fight for the job.
The former chancellor of the exchequer said his pro-European views could disqualify him.
Mr Clarke said he would wait for 10 days or so before deciding whether to stand against Mr Michael Portillo. Speaking outside his home in Kennington, south London, he said he had to consider whether he could lead a party which had just carried out a Eurosceptic election campaign.
"I would like to be leader of the Conservative Party and I have not given up my ambition yet to one day be prime minister," he told ITN.
"I have to decide whether a parliamentary party that has just fought a frenetic Eurosceptic campaign is really capable of being led by a proEuropean. That requires some careful consideration with many members of the parliamentary party."
Mr Portillo, who is the favourite to succeed Mr William Hague, on Wednesday became the first Tory to declare his candidacy. Other senior Conservatives, including the defence spokesman, Mr Iain Duncan Smith, and shadow home secretary, Ms Ann Widdecombe, are still considering their options.
Mr Portillo's declaration prompted an immediate rebuff from Ms Widdecombe, who declared she could never serve under him as leader. The Tory vice-chairman, Mr Tim Collins, admitted sweeping changes were needed in the wake of Labour's election landslide. Mr Collins, a key election campaign strategist, offered qualified encouragement for Mr Portillo.
"The voters are our masters, the voters rejected us," he said. "The voters are right and therefore we were wrong and we need to change a lot of things about the party, both in terms of our organisation and our approach, the way we look.
"Michael Portillo and others are absolutely right to say that," Mr Collins told BBC Radio 4's World at One. But he stressed that the Tories should not reject everything they campaigned for out of "panic" in the wake of the crushing defeat, but build on the "strong foundations" laid over the past four years.
Mr Collins called Mr Portillo "an extremely strong candidate for the leadership", but added that he had not decided who he would like to see as the next Tory leader. Meanwhile, the Conservative MP Mr Ian Taylor warned that his party was in danger of being pushed into third place behind the Liberal Democrats.
Mr Taylor, a close ally of Mr Clarke, said he would like the former chancellor to be party leader but was realistic about his chances.
The former trade and industry minister added: "The Tory Party is in danger of becoming a third party. We are at the very dangerous steps and there is no guaranteed bounce-back theory. At this election we were really defeated - last time we were just rejected."
On Mr Clarke's chances in a leadership battle, Mr Taylor remarked: "I am not putting him forward because he is proEuropean. That happens to be an attribute, but I recognise that it could be a problem for the Tory party. I am putting him forward because he is head and shoulders above other people.
"I would like him to lead but I am not going to head count. Support is either instinctive or not. Ken is not going to win by grabbing for every backbench colleague."