But unity is in short supply in the U.K.'s fractious, anxious governing party — and Johnson's vow of loyalty did not quell suspicions he covets the leadership.
Johnson told delegates to the Conservative annual conference that "the whole country owes (May) a debt for her steadfastness in taking Britain forward, as she will, to a great Brexit deal."
May laid out her plans for Britain's exit from the European Union in a speech last month in Florence — "on whose every syllable I can tell you, the whole Cabinet is united," Johnson said.
Johnson has spent weeks giving the opposite impression. He has been accused of undermining the prime minister — and advancing his leadership ambitions — by laying out his own distinct roadmap for Britain's exit from the European Union.
With EU divorce negotiations proceeding at a snail's pace, Johnson has positioned himself as a champion of a clean-break "hard Brexit." He wants the U.K. to adopt a low-tax, low-regulation economy outside the EU's single market, says Britain must not pay to get tariff-free trade with the EU and insists that any post-Brexit transition period should not last "a second more" than two years.
Johnson told conference delegates that it was time to stop being negative about Brexit and "treating the referendum result as though it were a plague of boils."
"It is time to be bold, and to seize the opportunities, and there is no country better placed than Britain," he said.
Johnson's Brexit stance — tougher than May's stated position — has added to uncertainty for British businesses, who want to know whether they will keep easy access to the EU market and its population of nearly half a billion.
The British Chambers of Commerce warned that businesspeople "are growing impatient with division and disorganization at the heart of the party of government."
German politician Manfred Weber, head of the biggest party group in the European Parliament, implored May on Tuesday to fire Johnson, "because we need a clear answer who is responsible for the British position."
U.K. finance minister Philip Hammond issued a warning to Johnson, telling broadcaster ITV: "Nobody is unsackable."
But May's power to silence Johnson is limited. Earlier this year she called a snap election in hopes of boosting the Conservative majority in Parliament and strengthening her hand in Brexit negotiations. But after a lackluster campaign that saw her dubbed the "Maybot," voters reduced the Conservatives to a minority administration.
May's speech wrapping up the conference in Manchester on Wednesday will be a make-or-break chance to bring the party into line.
May insisted Tuesday she was firmly in charge, saying "weak leadership is having a Cabinet of yes men." She has vowed to lead her party into the next election, due in 2022.
But many Conservatives see her as a lame duck, and a leadership challenge could come much sooner.
"All she can do is earn (the party's) tolerance for another year or two, until such time as they decide that it's time for her to go," said Tim Bale, professor of politics at Queen Mary University of London.
Johnson is not May's only rival. Some pro-Brexit Conservatives favor lawmaker Jacob Rees-Mogg, whose combination of social conservatism and tweed-suited upper-class assurance strikes his fans as a badge of political authenticity.
Another rising star is charismatic Scottish Conservative leader Ruth Davidson, who told delegates that the party "needs to get over its current nervous breakdown and man up a bit."
To add to May's woes, she and senior ministers feel they must use their conference speeches to defend capitalism against a resurgent British left.
The Labour Party under veteran socialist Jeremy Corbyn surprised many in June's election by gaining votes — especially among the young — with an old-school tax-and-spend platform of nationalizing key industries, increasing public-sector pay and boosting welfare benefits.
In response, the Conservatives have announced policies aimed at younger voters, including a freeze on university fees and a 10 billion-pound ($13.3 billion) boost to a housing help-to-buy program.
Many doubt these modest measures will do much to woo young people back from Labour and reverse what some fear is a terminal Conservative decline.
"It's a bloody mess," said Ian Smith, a party member from Shrewsbury in central England. "If I look at my local party, it's a joke. It's full of ancient people and hasn't got a clue how to campaign on the ground. ... I'm 62 and I'm like a youngster there.
"I hope the last election has been a wake-up call."
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