Chris Huhne has said new Liberal Democrat leader Sir Menzies Campbell is more than just a "caretaker".
Mr Huhne said he tried to put "new ideas" on the table.
Mr Huhne, who came second to Sir Menzies in a leadership ballot, pledged his "full support" to the new leader.
And said he saw no reason why the 64-year-old could not lead the party into two general elections.
Sir Menzies' victory - with 57% of the vote - was "a hard won and decisive mandate to lead this party to new advances", said Mr Huhne.
He told Lib Dem members that the "best days of the party are still to come".
The economics spokesman, who trailed Sir Menzies by 8,069 votes, was seen as a rank outsider when he entered the contest and has now put himself firmly on the Westminster map.
But press reports he had broken an agreement with Sir Menzies not to run against him were dismissed by Lib Dem frontbencher Sandra Gidley as being "puffed-up" by the media.
She said the party would put such "silliness" behind it and get behind its new leader.
In the second and final round of the contest he secured 21,628 votes to Sir Menzies 29,697, after party president Simon Hughes was knocked out in the earlier round.
Speaking as the result was announced, the former MEP said: "This party needed a real contest and that's exactly what we've had. The result means very clearly that Ming has a hard won and decisive mandate to lead this party to new advances with the backing of all of us."
"I am more convinced than ever, after this contest, that the best days of this party are still to come; that we are poised for a further substantial advance that will take Liberal Democrat principles into power," he added.
"All good luck to our new leader", he said, saying the cause, which he and Sir Menzies cared passionately about was in "very good hands".
He later told BBC News 24: "What I tried to do in this contest is put some new ideas on the table which I think we need to really campaign on, and particularly about a style of campaigning against the other parties.
"We need to have a hard edge to our policies, so that when somebody like David Cameron comes over the hill saying "I care about the environment and global warming" we can turn around and say 'well hang on a minute, it's not enough just to come up with warm words.
We need concrete proposals and if you're not prepared to make those you shouldn't be part of this game' ".
Addressing Mr Campbell's age, which was an issue during the campaign, he said he did not see the new leader as a "caretaker".
MP, Susan Kramer, who backed Chris Huhne, said she was not unhappy with the final result.
"Ming is going to be a tremendous leader," she said.
"So I think this is going to be a good result for everybody - and you're right, Chris (Huhne) did us proud."