Shadow cabinet member Theresa May has backed David Cameron to be the next Conservative Party leader.
May, left, is backing Cameron, right
Mrs May, who had been considering standing herself, said: "My view is that David Cameron understands the depth of change which is needed."
She spoke as the four candidates prepared to address a meeting of about 60 right-wing Conservative MPs.
Sir Malcolm Rifkind quit the contest on Tuesday and backed Ken Clarke. Liam Fox and David Davis are also standing.
Manager of Iain Duncan Smith's successful leadership campaign in 2001, Bernard Jenkin, has also declared for Mr Cameron.
The shadow energy minister said Mr Cameron was "a very cool cucumber" who seemed "completely unfazed...by extraordinary media attention".
The hustings - one after the other rather than all at the same time - is the first of three similar events ahead of the first round of Tory MPs' voting next Tuesday.
On Thursday they will face the Contact Group of Conservative spouses, while next Monday there is expected to be a hustings in front of the full parliamentary party.
Mrs May told the BBC that she thought Mr Cameron was the best candidate to help win back the support of young people and women, both groups where support had been lost.
She said voters liked Tory policies until they discovered they were Tory ones.
"I am not suggesting we throw away our core principles and beliefs," she said.
She said Mr Cameron offered "a fresh approach for the Conservative Party".
Wednesday: Hustings in front of right-wing Tory MPs from the 92 Group, No Turning Back Group and Cornerstone Group
Thursday: Nominations close. Hustings for the Contact group of Tory spouses
Monday: Possible hustings for all Tory MPs
Tuesday: First round of voting by MPs
Early November: Tory members start voting on final two candidates
6 December: Result expected
Sir Malcolm Rifkind pulled out of the race to replace Michael Howard as Conservative Party leader on Tuesday to back Mr Clarke.
Sir Malcolm, a rank outsider in the race, told BBC News he had won far less support than he would have liked.
"There is no realistic prospect of me coming through," said the 59-year-old shadow work and pensions secretary.
He said Mr Clarke was "head and shoulders" above the rest as a potential prime minister.
His departure means that by Thursday next week MPs should have whittled down the choice of candidates to the two who will go to a ballot of all Tory members.
Thursday's deadline for nominations will be the last chance for any surprise candidates to put themselves forward.
The Cornerstone Group has announced it will not field a candidate, saying it will decide whether to recommend one of the existing candidates after seeing them in action at Wednesday's hustings.
The 92 Group and the No Turning Back Group are the others who will attend Wednesday's meeting.
The first round of voting by Conservative MPs is next Tuesday, with the candidate receiving the fewest votes being eliminated.
The process will be repeated two days later.
The estimated 300,000 Conservative members nationwide then have the final say, with a result due on 6 December.