After a string of bad knocks including a widely derided performance at last week’s presidential debate, Jeb Bush vowed yesterday to “reset and get better” beginning with a major speech in Tampa, Florida, that will attempt a newly positive tone.

“We need to be hopeful and optimistic, have an aspirational message,” Mr Bush said in an interview with NBC News previewing Monday's speech. “I don’t think conservatives are going to win the presidency unless we campaign with our arms wide open.”

Yet optimism is a commodity in short supply for Mr Bush. Even before stumbling at the debate, when a pointed attack on Senator Marco Rubio – fast emerging as perhaps his most potent rival – boomeranged on him, Mr Bush had been forced to slash his campaign amid dwindling love from donors, some of whom have started to voice doubts about his chances of winning the Republican nomination.

Mr Bush also faced allegations that his campaign had deliberately leaked an internal memo detailing its game plan for deflecting Mr Rubio. The issue is delicate because campaigns and outside political action committees, or Super-PACs, are not allowed to co-ordinate strategy. But once such a memo is in the public domain, any group supporting Mr Bush will obviously be free to scrutinise it.

“I didn’t see it. I read about it when it was leaked for sure,” Mr Bush somewhat awkwardly protested to NBC. He said he was focused instead on winning the early-voting states like New Hampshire, Iowa and Nevada. “That’s our first mission,” he declared.

He also acknowledged his campaign’s difficulties. “I have enough self-awareness to know that this is the bumpy time of a campaign. This is the process. I totally understand it, and I’m more than prepared to fight on,” he said, adding: “I know that I got better at doing the debate… I mean, when I see that I’m not doing something well then I reset and I get better.”

For his part, Mr Rubio has not only basked in glowing reviews for his debate performance, but also enjoyed a significant boost on Friday when one of the biggest donors to the Republican Party, Paul Singer, gave him his backing. His decision was another disappointment for Mr Bush, who had been courting him but also for others in the field hoping for his support, such as Chris Christie, Governor of New Jersey.

In a letter to other Republican activists, Mr Singer, who has been a rare voice in the party supporting gay marriage, suggested that Mr Rubio, 44, who was once a protégé of Mr Bush, was the only candidate who can “navigate this complex primary process, and still be in a position to defeat” Hillary Clinton in a general election next year. “He is ready to be an informed and assertive decision-maker,” he added.

Among the most noted lines from the Bush campaign memo was that Mr Rubio may be a “Republican Obama”. Mr Rubio said yesterday he did not consider the description a compliment. He meanwhile added that if Mr Bush wants to fire arrows at him as he did in the debate – for allegedly neglecting his duties as US senator while on the campaign trail – he would not be returning then in kind.