Former US ambassador to the United Nations Nikki Haley announced Tuesday she is running for president, becoming the first high-profile candidate to challenge Donald Trump –- her onetime boss -- for the Republican nomination.

Casting herself as a younger, fresher alternative to the 76-year-old Trump, Haley had been hinting at a possible run for weeks and teasing a "big announcement" on February 15.

In the end, the 51-year-old declared her candidacy a day early, on Valentine's Day.

"I'm Nikki Haley and I'm running for president," the former governor of South Carolina and the child of Indian immigrants said in a video statement.

"It's time for a new generation of leadership -- to rediscover fiscal responsibility, secure our border and strengthen our country, our pride and our purpose," she said in the video shot in Bamberg, the South Carolina town of her birth.

Haley is positioning herself as a changemaker who can reinvigorate a party and country she says have lost their way in recent years, and she played up her personal background as part of her appeal to unite a nation strained by racial tensions.

"I was the proud daughter of Indian immigrants. Not black, not white. I was different," she said in her announcement video.

"But my mom would always say, 'Your job is not to focus on the differences, but the similarities.'"

"Some look at our past as evidence that America's founding principles are bad," she went on.

"They say the promise of freedom is just made up. Some think our ideas are not just wrong, but racist and evil. Nothing could be further from the truth."

Haley, who would be the country's first female president if elected, is unlikely to be the last Republican to throw their hat in the ring.

Some Washington watchers speculated her announcement might prompt a stampede from rivals such as Florida Governor Ron DeSantis, Trump's vice president Mike Pence and secretary of state Mike Pompeo, and former Maryland governor Larry Hogan.

Haley also took a swipe at the current president -- who has not formally announced his re-election campaign but is expected to run again -- saying, "Joe Biden's record is abysmal."

"But that shouldn't come as a surprise. The Washington establishment has failed us over and over and over again," she said.

Rocky ties with Trump

At this point in the 2020 election cycle, 10 Democrats had launched campaigns or exploratory committees, but Trump and now Haley are the only Republicans to do so officially this time around.

In her announcement she blasted China over its "genocide" of ethnic minority members, warned that Russia was "on the march," and accused Iran of "murder" of its own people for challenging the government.

Haley vowed to be tough with America's rivals.

"You should know this about me: I don't put up with bullies," she said. "And when you kick back, it hurts them more if you're wearing heels."

Haley rose quickly in South Carolina politics, building a reputation as a plain-spoken conservative.

She endorsed senators Marco Rubio and Ted Cruz during the 2016 Republican presidential primary, and slammed Trump as "everything a governor doesn't want in a president."

But after Trump won he tapped her as his UN envoy, and Haley became the face of diversity in a cabinet criticized for being too white.

Since leaving government in 2018 with a reputation for standing up to her mercurial boss, her occasional praise of the Trump presidency has been offset by her criticism of his personal conduct, including his involvement in the 2021 attack on the US Capitol.

Trump wished Haley luck in a statement to CNN on Tuesday, even as he took a jab at her for apparently going back on her word about running against him.

"Even though Nikki Haley said, 'I would never run against my President, he was a great President, the best President in my lifetime,' I told her she should follow her heart and do what she wants to do," he said. "I wish her luck!"

Haley averages under four percent in 2024 primary opinion polls, according to Real Clear Politics, trailing far behind Trump at 48 percent, DeSantis at 30 percent and Pence at seven percent.

The Democratic National Committee (DNC) was quick to state that Haley cozied up to Trump and "has embraced the most extreme elements" of his Make America Great Again agenda.

"Haley's entrance officially kicks off a messy 2024 primary race for the MAGA base that has long been brewing," DNC chairman Jaime Harrison said in a statement. "Everyone get your popcorn."