Haley’s decision to suspend her campaign comes after she outlasted 12 male Republican candidates for the presidential nomination.

Loading

Most of those – including Florida Governor Ron DeSantis, biotech entrepreneur Vivek Ramaswamy and South Carolina Senator Tim Scott – have since endorsed Trump, and some are now hoping for a place in a future Trump administration.

Haley, a two-term governor of South Carolina, began her bid for the nomination a year ago, treading carefully for fear of angering Trump’s base, but spent the past few months painting him and Biden as too old and mentally unfit for the presidency, and depicting Trump as a chaos agent with a history of losing general elections.

Haley’s departure kick-starts an almost certain rematch between the 81-year-old president and his 77-year-old predecessor, despite polls showing most Americans don’t want to endure such a contest.

While both men have not yet secured the required number of delegates they need to officially clinch the nomination, their near-clean sweep of the Super Tuesday primary and caucus races means that it is only a matter of time.

Biden, according to a strategy campaign memo, will use his State of the Union address on Thursday night (Friday AEDT) to articulate the “stark choice” voters face between his “historically popular record of accomplishment” and Trump’s “dark” and “dangerous” vision for the country.

Trump, meanwhile, used his speech on Tuesday night to pivot to the November 5 election, declaring it would be “the single most important day in the history of our country”. He also focused heavily on the issues central to his campaign: immigration, crime, and the “weaponisation” of the justice system that has indicted him four times on criminal charges.

While Trump now has an iron grip on his party, Haley’s performance in the primaries exposed weaknesses that could plague his attempt to return to power.

In the early voting states, Haley picked up a significant share of voters who remain resistant to Trump.

Loading

This included 43 per cent of voters in New Hampshire, 40 per cent in South Carolina, 27 per cent in Michigan, and 19 per cent in Iowa, when several other candidates, including DeSantis, were still in the mix.

On Super Tuesday, she also won a significant share of Republican primary and independent voters, including a victory in Vermont, and had earlier picked up Washington DC. But she was unable to secure anywhere near enough delegates to slow Trump’s momentum.

Both Trump and Biden will now seek to woo Haley’s voters in what is expected to be an extremely close race, where voter turnout could make all the difference.

“I said I wanted Americans to have their voices heard. I have done that. I have no regrets,” Haley, who has ruled out an independent run, said in her speech.

“And although I will no longer be a candidate, I will not stop using my voice for the things I believe in.” This, she said, included fiscal responsibility and standing by allies such as Ukraine and Taiwan, in “a world on fire”.

Haley’s refusal to endorse Trump also comes after she signed a pledge last year as a condition of taking part in the Republican debate to back whoever won the nomination.

In a sign of potential party unity, Senate Republican leader Mitch McConnell endorsed Trump for president on Wednesday.

However, Trump won’t be formally announced as the Republican nominee until the party’s convention in July, and some believe she could change her mind to shore up more support within the party ahead of the next election, or to be viewed as the alternative candidate if Trump was no longer able to run.

Explaining her decision, Haley said: “I have always been a conservative Republican and always supported the Republican nominee. But on this question, as she did on so many others, Margaret Thatcher provided some good advice when she said ‘Never just follow the crowd. Always make up your own mind’.”

Get a note directly from our foreign correspondents on what’s making headlines around the world. Sign up for the weekly What in the World newsletter here.