By Nick Assinder
Political Correspondent, BBC News website
Ken Clarke has never hidden his desire to become Tory prime minister - but will it be third time lucky?
After two previous, failed attempts to lead his party, the 65-year-old big beast of the Tory backbenches has formally announce his decision to join the battle to replace Michael Howard.
Clarke has ended months of agonising
His decision, after months of agonising, will inject some new energy and urgency into the battle.
It has already sent shudders through the other would-be leaders' camps, despite the fact they were expecting it. And it is likely to change the dynamics of the entire leadership race.
Front-runner David Davis will now be bracing himself for a powerful left-wing challenge from the man seen by a large number of backbenchers as their best hope for the next election - and their own futures - and the most likely to deflate Tony Blair and, more importantly, future prime minister Gordon Brown.
David Cameron, whose supporters still insist will run, will have to calculate his chances of winning the party's modernising vote in the face of the challenge from the most liberal, if least youthful, of his colleagues - and whether he can risk splitting that vote.
And, despite his determination to fight on, Sir Malcolm Rifkind may be wondering whether it really will be worth continuing to the bitter end now there is a bigger name carrying the One Nation banner he supports.
Even before Mr Clarke's announcement, Tim Yeo had already followed that course.
Meanwhile other contenders which could include Liam Fox, Theresa May, David Willetts and Andrew Lansley will all be going back to their canvassers to check just how many of their previous pledges of support may now start drifting away.
This is because many in the Tory party believe Mr Clarke's decision means this leadership battle is now certain to end as a straight fight between him and Mr Davis.
And those determined to stop Mr Davis - and this is undoubtedly part of Mr Clarke's calculations - may decide to fall in behind the former chancellor, even if some harbour doubts about his enthusiasm for Europe.
Yeo stood down to back Clarke
Mr Clarke went some way towards calming those jitters with a recent speech in which he pronounced the EU constitution dead and the single currency a failure.
He went further in his newspaper interview launching his bid, declaring the euro would not happen in Britain during his political life time.
In other words, while he remains pro-EU, he would not use his leadership to push a fiercely Euro agenda.
And he must now believe that was enough to give his third leadership bid a real chance of success.
Meanwhile, there has been renewed talk of a dream ticket with calls for Mr Cameron to put his ambitions on hold to stand as Mr Clarke's running mate - an echo of the alleged Blair-Brown Granita deal,
But it is still a little too early to be closing down other options.
The biggest unknown is over what procedure the party will finally adopt to elect the leader.
If, as expected, it is left in the hands of Tory MPs, Mr Clarke will feel more confident and it is likely some candidates will be persuaded not to stand, or abandon their challenges.
Davis will face challenge
He may have failed to win over MPs' in his first leadership bid, after John Major quit. But most put that down to his stand on the EU which may not be as big an obstacle now.
On the other hand, if the vote goes to the membership, Mr Davis will feel more confident.
That is the system which saw Mr Clarke defeated at his second leadership attempt by Iain Duncan Smith, again over Europe.
Whatever the outcome, however, it is also likely the leadership contest will now enter a new, more aggressive phase.
His opponents will point out Mr Clarke's age - he will be close to 70 at the next election - and claim that, while he is hugely experienced and capable, the Tories have moved on since he was last in a cabinet.
There will also be doubts again raised over his support for Europe.
The biggest platform for the contenders will come at the party conference in Blackpool in October.
And it is there Mr Clarke will get the clearest sign yet of whether three really is going to be his lucky number.