Here's a look at her policy positions and how they differ to Trump's.
Abortion
Haley supported restrictive abortion measures as a South Carolina lawmaker and later governor, including a 24-hour "reflection period" for women considering the procedure. She also backed ending abortion coverage for rape and incest victims in the state health plan, although this didn't pass.
But during the presidential campaign she has cast herself as a reasonable Republican, insisting her party requires "consensus" before attempting to pursue a national abortion policy.
She also says birth control should remain legal and women shouldn't go to prison or get the death penalty for terminating pregnancies -- stances meant to endear herself to independents, with Democrats set to run hard on protecting abortion rights.
Trump has campaigned hard against abortion and touts the Supreme Court's ending of the constitutional right to the procedure as a major achievement.
Race
The daughter of Indian immigrant parents, Nimarata Nikki Haley -- who goes by her middle name meaning "little one" in Punjabi -- has a complex relationship with race.
In her 2012 memoir, Haley detailed the racism she and her family faced growing up in the rural South, where her father's Sikh turban stuck out and the Randhawas had to buy a house as no one would rent to them.
As governor, she supported flying the Confederate flag on state buildings until a mass shooting at a Black church made her reverse course.
Her recent comments that the United States has "never been a racist country," and that the Civil War was fought over state's rights, rather than slavery, triggered widespread outrage.
Climate
Haley acknowledges the reality of human-caused climate change, unlike Trump who has branded it a "hoax."
Beyond the rhetoric, there is little daylight between them.
As Trump's UN ambassador, she helped remove the United States from the landmark Paris Agreement. Her proposed tax plan involves repealing hundreds of billions of dollars in green energy subsidies under President Joe Biden's signature climate law, calling it a "communist manifesto."
She has also promised to speed up fossil fuel drilling and weaken the Environmental Protection Agency.
Immigration, LGBT rights, economics
Echoing Trump, she has called on the United States to "close" its southern border and defund "sanctuary cities."
She has controversially proposed sending the military into Mexico to target drug cartels.
On social issues, she has said an infamous Florida education law that prohibits classroom discussions on sexual orientation or gender identity through third grade doesn't go "far enough," and the ban should extend throughout elementary school.
She has proposed increasing the retirement age for younger workers while maintaining retirement payments and state-subsidized healthcare for those approaching retirement. Though this was until recently standard conservative fare, it has come under fire from Trump whose populist politics has shifted some old left-right splits.
Foreign policy
As Trump's UN ambassador, Haley made supporting Israel her signature issue, blocking the appointment of a Palestinian envoy, and helping quash a UN report that accused Israel of "apartheid."
She recently said Palestinians should be sent from Gaza to "pro-Hamas" countries.
Trump has frequently expressed admiration for Russian President Vladimir Putin. He also claims, without ever explaining how, that he could bring an end to fighting in Ukraine, which was invaded two years ago by Russia, in "24 hours."
While Trump is an isolationist who believes US allies are "ripping off" Washington, Haley backs US military aid to Kyiv, painting this as support for democracy, but also part of a broader US national security plan.
January 6
Perhaps the clearest split between Trump and Haley are their positions on the storming of the Capitol on January 6, 2021, which Haley called "a terrible day" while acknowledging that Biden won the 2020 election.
At the same time, she has indicated she would support Trump if he won the party's nomination, even if he is convicted of a felony.
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