In a sideswipe at David Davis and Liam Fox, the main challengers from the right, the former chancellor claimed that he was the only candidate who could win voters from Labour and the Liberal Democrats.
"My party has to make a decision: do we want to remain in opposition or are we going to get serious and pitch for government? I say to my fellow Conservatives: do not elect a man who wants to be leader of the opposition. Elect a leader who wants to be prime minister,'' he said.
The choice was stark - government or opposition. "It's time for the Conservative Party to unite around a leader who can lead us to victory,'' he said.
His attack on the other candidates for dooming the Tories to perpetual opposition will upset his opponents and could herald a bitterly fought contest. It is likely to be early December before a new leader emerges.
Stuart Wheeler, a major donor to the party, said he would hesitate before giving money to a Clarke-led Conservative Party. He said Europe remained a crucial issue and was worried Mr Clarke would not put his heart into getting major changes to the European Union.
Mr Davis and David Cameron, who is standing as a "one-nation" candidate like Mr Clarke, will launch their campaigns today. They will turn next week's Tory conference in Blackpool into a week-long hustings and the candidates will speak from the platform.
Mr Clarke's main task is to win over enough MPs to make sure he is on the shortlist of two candidates to go before party members for a secret ballot. Many MPs have privately said they would vote to stop Mr Clarke going into the final ballot, but there will be an outcry from the country if Mr Clarke fails to reach the run-off.
He has previously suggested that the party's grass roots were too right-wing to elect him as leader. But he said: "I have increasingly sensed a great change of mood among the voluntary membership of the party."
He now believed there was an "overwhelming" desire among activists to ensure the Conservatives were returned to power. The MP for Rushcliffe, in Nottinghamshire, described himself as a "provincial man" and pointed to polls suggesting he was the most popular candidate in all regions and across all groups.
Mr Clarke, at 65, is by far the oldest of the candidates but will try to counter claims that he is too old to be leader by appealing to younger supporters. Mr Clarke, who lost out on the leadership in 1997 and 2001, said he was the "original moderniser".
Meanwhile, David Davis, the front-runner to replace Michael Howard, was meeting party activists in Wales.
Mr Cameron, at 38, the youngest candidate, will highlight the age gap today by committing himself to reforming the Tory party with "fresh ideas". He will say: "Britain aches for new hope, fresh ideas and reform with results. A Conservative government can provide these things. But first we must be honest with ourselves: we can only change our country if we dare to change our party. No more marking time. No more false starts. No more bewildered retreat to our comfort zone. We need bold, confident and consistent leadership - leadership that recognises we must change to win."
Nominations for the contest open on 10 October - after the party's conference - with the first round of voting among MPs taking place on Tuesday 18 October.