LONDON — Boris Johnson is the bookies' favorite to succeed Theresa May — but don't bet on it.

Previous leadership contests for the U.K.'s Tory party suggest that the title of "front-runner" — the tag currently attached to the former foreign secretary — is more hindrance than help.

It draws fire from other contenders and brings disproportionate scrutiny from the media early in the race, as well as from what one veteran described as the "duplicitous electorate" of Tory MPs.

"Do the favorites win? History says very rarely," said Steven Norris, a veteran former Conservative MP.

Here is POLITICO's guide to the Tory leaders that never were, and the factors that led to their undoing after promising so much at the start of their leadership campaigns. The form book shows that Tory leadership elections often produce shock results.

The aborted contest

The most recent Tory leadership contest, in 2016, was triggered when David Cameron stood down the morning after the Brexit referendum result.

Johnson, who was the early front-runner three years ago, dropped out unexpectedly after his campaign manager Michael Gove, a fellow leading light in the Leave campaign, announced he had decided to stand himself.

The field was then quickly whittled down to just May and Andrea Leadsom, a previously little-known energy minister who had also featured prominently in the campaign to leave the European Union.

Leadsom stood aside after a disastrous interview in the Times in which she said that having children means she had "a very real stake" in Britain's future. The comment came days after May had spoken about her sadness at being unable to have children and it was widely regarded as a callous dig by Leadsom.

The long campaign

David Davis, the Brexit secretary until July last year, was the early front-runner in the 2005 contest to replace Michael Howard, who announced he was standing down following another Tory election defeat. Seven months later the lesser-known Cameron had wowed the Tory parliamentary party and faithful, and became the leader of the opposition and subsequently prime minister in 2010.

Many commentators believe a quick leadership election would have favored Davis, who had lots of MP backers, but the lengthy contest gave Cameron a chance to build support among MPs and the membership. It was a memorable and passionate no-notes speech to party conference that really sealed the deal.

"To the membership, Cameron probably was a relative unknown," said the Institute for Government's historian Catherine Haddon. "Reportedly it was that one speech that really turned the eyes of MPs which got him through to the second ballot. It was the membership vote and his campaigning there that made the difference, which was why he stormed ahead of David Davis ... but it was also about David Davis' own performance, and what was considered a lackluster speech from him [at the same party conference]."

Johnson supporters will hope that the current race can be over quickly (perhaps if several in the crowded field pull out early) to prevent another contender building momentum.

Power of the membership

A change in the leadership election rules in 1998 — which involved the party's members for the first time in a final round selection — produced a shock result in the 2001 contest.

The final round of voting among MPs was close. Europhile front-runner Ken Clarke won 59 votes, with Euroskeptic Iain Duncan Smith on 54. Michael Portillo was eliminated on 53 votes, despite being ahead in the previous two ballots. In the all-member runoff, Duncan Smith defeated Clarke by 61 percent to 39 percent.

"Ken Clarke was infinitely better known and more experienced than Iain Duncan Smith, who was almost unknown at the time, but appears to have been elected because he was not Ken Clarke," according to veteran former Tory MP Norris, who thinks Portillo would almost certainly have won had he "bothered to engage."

"[Portillo] was seen as a very attractive candidate, good speaker, great intellect and yet he dropped out by one vote. He certainly didn't bother to talk to colleagues when everybody expected that he would. It is yet another of those real surprise results which shocked people at the time," Norris added.

Tactical voting

The 1997 leadership election was another contest that was not to be for Clarke. It followed a landslide win for Labour's Tony Blair at the general election, which led to several Tories seen as potential leaders — including Ian Lang and Malcolm Rifkind — losing their seats. Ousted Prime Minister John Major stood down as Tory leader. The contest was fought under the previous rules — a ballot solely of Conservative MPs without any direct involvement for members.

Euroskeptic heavyweights — John Redwood, Peter Lilley and Michael Howard — all stood, as did Clarke and the ambitious young William Hague. "Everyone assumed that Michael Howard would succeed John Major and yet he didn't. It was the — almost invisible at the time — William Hague," Norris said.

The final runoff was between Clarke and Hague. Supporters of Redwood wanted to stop Clarke becoming leader and so gave their votes to Hague. So although Clarke's vote increased from 64 to 72 in the third ballot, Hague's jumped from 62 to 90, meaning he took the crown.

"The crucial thing with the MPs' ballots, it is not really about whose support [you have], but more about who you are willing to join forces with to try and stop a particular candidate," said Haddon.

That could end up being relevant in the current leadership contest. "It is whether or not there is a sufficient 'stop Boris' campaign and whether or not a candidate might slip through because of that," Haddon added. "That is where the big issue can come."

Compromise candidate

The contest to replace Margaret Thatcher as leader was triggered by a challenge from her former Defense Secretary Michael Heseltine.

This race contained two big surprises. First, most MPs assumed that Thatcher would see off the challenge, and yet she didn't. She won 204 votes in the first ballot to Heseltine's 152 — not a big enough margin to prevent a second round. After initially declaring, "I fight on; I fight to win," she subsequently withdrew from the race.

"If you had added together all the people who had said they had voted for Mrs Thatcher in 1990 she couldn't conceivably have lost. I think similar things may be happening [in the current race]," said Norris.

The second surprise was the eventual winner. With Thatcher stepping aside, Major then stood against Heseltine, who was seen as a divisive figure by many in the Cabinet. In the second round, he secured the backing of some on Thatcher's right wing of the party and finished far ahead of Heseltine and the other challenger, former Home Secretary Douglas Hurd. That led Heseltine and Hurd to pull out.

Though Major was chancellor and appeared to be Thatcher's anointed successor when he stood, Haddon points out that he was not the "obvious candidate that they would have all spoken about the week before Thatcher resigned. He is the archetypal compromise candidate."

To the traitor the spoils

Nobody would have predicted Thatcher herself would take over as leader of the opposition back in 1975 when the party's previous leader Edward Heath called a contest to shore up his position. With key shadow Cabinet figures pledging their support, he calculated (wrongly) that it was a safe gamble because there was no obvious challenger.

Thatcher stood against him in the subsequent leadership election, and successfully tapped into the votes of disgruntled backbenchers to take the prize.

Smoke-filled rooms

Before 1965, leaders of the Conservative Party were not elected, but emerged after a behind-closed-doors discussion among senior Tories.

In the process (it is hard to call it a contest) to replace Harold Macmillan in 1963, who retired because of ill-health, the front-runner was widely regarded to be Rab Butler. He had held three of the four great offices of state (chancellor, home secretary and foreign secretary). But unelected peer, and family friend of Queen Elizabeth II, Alec Douglas-Home emerged as the winner.

In an essay, published in an anthology of British Conservative leaders, academic Andrew Holt says the extent to which MacMillan was determined to avoid Butler succeeding him was still debated, but quotes Douglas-Home biographer David Dutton, who says it is hard to escape the conclusion that MacMillan was determined to avoid having Butler succeed him.

The coronation

One exception to the rule was the election of Michael Howard in 2003, a contest which was less leadership race and more coronation. He emerged as the unity candidate after Iain Duncan Smith's disastrous spell as leader ended in 2003. He was anointed by the parliamentary party without a contest.

Howard's name "had been in the frame," said one veteran Tory MP, even though he did not have a good reputation among some MPs. But fearing a bloody battle that could further harm the party's standing with voters, other potential contenders were prepared to back off and give him a clear run.

Boris Johnson's star power and popularity with the Tory grassroots could well see him through — if he can make it to the final runoff

"The party by 2005 had been out of power for eight years, it had just lost its second general election. There was this sense, as there was with May, this is the obvious person so why are we bothering to put a second candidate up when nobody is really going to stand against Michael [Howard]," the Tory veteran MP said.

Hope for Boris?

Other candidates in the current leadership race are already playing up the curse of the front-runner. When challenged on the BBC's "Today" program that Johnson is way out ahead of the pack, Matt Hancock, the health secretary and another contender, shot back: "Well the front-runner has never won before has he."

Norris, who does not want to see a Johnson victory, said he is optimistic the tide in the race will shift. He said he is “buoyed up by the fact “ that so many of the 313 Tory MPs are still to declare. "It is a secret ballot so many people may say who they are going to vote for looking for a job, but how they actually vote is another matter," he added.

But according to Haddon, the main hope for Johnson is the highly unusual nature and size of the contest to succeed May. "It is quite extraordinary, it is difficult to look at decent comparisons," Haddon said, adding there is a "good chance this one will play completely differently to any other."

"None of [the previous leadership races] had similar circumstances to this, not in terms of the circumstances when they are taking over, the divisions in the party that they are in theory either going to bring together or fail to, nor the number of candidates," she said.

In such a crowded field, Johnson's star power and popularity with the Tory grassroots could well see him through — if he can make it to the final runoff.

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