Boris Johnson’s resignation turns Theresa May’s manic Monday from a crisis about Brexit policy into a crisis about the Conservative leadership. The resignation is not about the former. It is about the latter. David Davis resigned overnight because he disagreed with May’s policy on customs and trade links with Europe. Johnson has now followed him because he wants to become prime minister. Davis resigned on an issue of principle; Johnson resigned on an issue of self-interest.

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That does not mean it will happen. Johnson is damaged goods in large parts of the Conservative party. Many will be delighted to see him go. He has been a second-rate foreign secretary. He bottled the issue on Heathrow, preferring to spend the day in Kabul rather than to vote the way he had said he would on the third runway. He has praised Donald Trump, even as the US president set about destroying global rules and alliances. He has lost some of the allure that he once had as the Heineken Tory who could reach parts of the electorate that other Tories couldn’t. In some parts of the party he is now an embarrassment. In a recent poll of Tory members on May’s successor, Johnson polled fourth, behind Sajid Javid, Michael Gove and Jacob Rees-Mogg.

The one thing that has not changed, though, is Johnson’s ambition. He may have a second-class mind and character, but he has a first-class ego. Even after he pulled out of the last leadership contest in 2016, he continued to see himself as a political figure above the prime minister and above the party. He can’t help himself in this respect. It’s simply the way he is. His self-regard is incontinent, just like Trump’s.

The threat to May is now severe. But it is not necessarily fatal. She faces a torrid next few hours in the House of Commons and then in a private meeting with Tory MPs. She has many cards in her hand, not least the support of a clear majority of her party at Westminster. A display of strong leadership may enhance her status, not diminish it still further. The public likes strong leaders who take their divided party by the scruff of the neck and tell them home truths. Whether May has the ability to do this will be pivotal too.

The question today, however, is whether the Brexiteers have now made the Conservative party ungovernable. The coming days will decide on that.

• Martin Kettle is a Guardian columnist