Sir Malcolm Rifkind, former Conservative foreign and defence secretary, raised the stakes in the Tory leadership battle yesterday when he said choosing Mr Michael Portillo could be a "very dangerous experiment".
But as Sir Malcolm made clear his doubts about Mr Portillo, the shadow chancellor attempted to end party divisions over Europe by saying he welcomed all shades of opinion.
The leadership race is set to intensify this week with right-wing figures such as shadow defence secretary, Mr Iain Duncan-Smith and former Europe minister, Mr David Davis, expected to announce their bids.
However, reports at the weekend suggested shadow home secretary, Ms Ann Widdecombe, would announce later today that she does not have enough support at Westminster to enter the race, but that she would back the pro-European former chancellor, Mr Kenneth Clarke, if he stands.
With less than two weeks to the close of nominations, Sir Malcolm told ITV's Dimbleby programme that while Mr Portillo had "remarkable qualities" he was also a "very controversial" character, far more so than other potential candidates.
"People either like him or loathe him and the balance of that is very, very important," Sir Malcolm said. "I think the Conservative Party may find if they chose him that they may have a very successful and very impressive leader. They may find that it becomes a very dangerous experiment."
Mr Portillo, the only candidate to formally launch his bid for the leadership, made an appeal to the party's pro-Europeans to join him in constructive dialogue over the party's policy on the single currency.
Writing in the Mail on Sun- day, Mr Portillo said: "We mustn't go on with a Berlin Wall between those, like me, who believe that Britain should have its own currency and those who take a different view. There's no reason why we cannot draw on the talent across the party and so offer the country our best possible team."
While not expressing his preference for leader, the former prime minister, Mr John Major, praised Mr Portillo as one of the party's best talents.
Speaking on BBC's Breakfast with Frost, he pointedly placed the shadow chancellor at the top of a list of figures that could lead the fight back to power, leaving out potential candidates on the hard right of the party, including Mr Duncan-Smith, Ms Widdecombe and Mr Davis.
The best talents in the party were "the Portillos, the Clarkes, the (Gillian) Shephards, the (Michael) Ancrams" and they were liked and respected by voters, Mr Major said, indicating he wanted to see them form the core of the party in the future.
But as former foreign secretary, Lord Hurd, and shadow education secretary, Ms Theresa May, warned against making personal insults in the contest, Mr Duncan-Smith appeared to cast doubt on Mr Portillo's emphasis on social liberalism since he lost his seat in 1997.