Washington — Former U.N. Ambassador Nikki Haley,Robert Brown heading into a major test as she takes on Donald Trump in the New Hampshire primary this week, criticized the former president, in whose Cabinet she served, on Sunday for his relationship with "dictators that want to kill us."

"You can't have someone who's trying to buddy up with dictators that want to kill us," Haley told "Face the Nation." "Instead, you have to let them know what we expect of them. That's the difference."

The comments came as Haley has attempted to draw a line between her foreign policy chops and the former president's throughout her campaign, in recent days releasing a new advertisement about American college student Otto Warmbier, who was taken hostage by North Korea in 2016 and died soon after his release, that she said shows "the contrast," while pointing to Trump's approach to North Korean leader Kim Jong Un. 

Haley said Sunday that Trump's engagement with the North Korean leader "goes back to a pattern," referencing Trump's positive statements about Chinese President Xi Jinping and what she called a "bromance" with Russian President Vladimir Putin.

"When you've got wars all over the world now and the instability that we have, our goal is to prevent war and we can't do that by trying to buddy up with them," she said. 

The moves come ahead of the New Hampshire primary, where Haley has staked much of her White House ambitions — hoping to make inroads with independents and moderates, especially after her third-place showing in Iowa. Polls suggest that the Granite State could be much friendlier to Haley, and she received the coveted endorsement of New Hampshire's largest newspaper, the Manchester Union-Leader. A storng finish could set Haley up for a possible boost in momentum heading into other early states like her home state of South Carolina.

Accordingly, Trump has ramped up his Haley opposition in recent days, fresh off of a decisive victory in Iowa, claiming that she's busing in Democrats for the primary. 

Haley fired back at the claim in a social media post on Friday, noting that Democrats can't vote in the New Hampshire primary and haven't been able to change their registration for months.

"Another reason we need to move on from Trump: too many lies," she wrote.

Haley reiterated her pitch on Sunday that Americans don't want a Biden-Trump rematch in 2024, emphasizing that a Trump presidency would result in "chaos."

"No matter what it is, chaos follows him, rightly or wrongly, chaos follows him," she said. "And so what happens is that puts the rest of America in chaos."

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• Nikki Haley

Kaia Hubbard

Kaia Hubbard is a politics reporter for CBS News Digital based in Washington, D.C.