Nikki Haley, as governor of South Carolina in December 2012, appointed Tim Scott to the Senate. Nearly 11 years later, on Wednesday night, Ms. Haley said he had squandered repeated opportunities to rein in spending. Mr. Scott said Ms. Haley had never seen a federal dollar she didn’t like.

“Bring it, Tim,” Ms. Haley said, taunting him from across the Republican presidential debate stage.

Nervous laughter erupted from the friendly audience as two South Carolinians seeking the Republican presidential nomination finally shed the shared Southern politesse that had kept them from attacking each other on the campaign trail.

Their skirmish began when Ms. Haley dismissed Mr. Scott’s promise to limit spending in Washington by pointing out the increase in the national debt during his time in the Senate.

“Where have you been?” Ms. Haley asked. “Where have you been, Tim? Twelve years we’ve waited, and nothing has happened.”

A few minutes later, Ms. Haley couldn’t contain her smile as Mr. Scott slowly wound up his counterattack, which fully unleashed their most vigorous exchange toward the end of an otherwise wearisome two-hour Republican debate.

She grinned, watching him as he spoke. She stole a glance into the audience, raising her eyebrows as if to acknowledge that the moment was as unavoidable as it was preposterous.

In past elections, Mr. Scott and Ms. Haley had campaigned together. Now, the former political allies were pitted against each other — bickering over the cost of gas and the price of drapes in a government office — in the increasingly desperate fight over second place to the race’s front-runner, former President Donald J. Trump.

Mr. Scott, whose sunny disposition typically casts him more naturally in the role of a happy warrior on the campaign trail, tried on the persona of a political brawler. It was an imperfect fit, and he stumbled over his words, stammering as the accusations trickled out.

As he turned to directly address Ms. Haley, he found her gaze waiting for him.

Their eyes met, and they nearly broke character, sharing the briefest of smiles — while trying to level criticisms at each other — and signaling the absurd twist that their longtime political alliance had taken.

“You literally put $50,000 on curtains at a $15 million subsidized location,” Mr. Scott said, waving his hand at Ms. Haley. It was a reference to a State Department allocation — made during the Obama administration and not by Ms. Haley — for $52,701 for the installation of customized window curtains in the high-rise apartment for the U.S. ambassador to the United Nations.

But it wasn’t immediately clear that Mr. Scott was finished. “Next,” he said quickly but awkwardly, suggesting that she could respond.

“You got bad information,” Ms. Haley said, emphasizing her adjective with a long drawl and wagging her left index finger at him first and then following it with a wag from her right.

She then defended herself against Mr. Scott’s accusations that she pushed to increase the gas tax, saying, “I fought the gas tax in South Carolina multiple times against the establishment.”

“Just go to YouTube,” Mr. Scott interjected. “All you have to do is watch Nikki Haley on YouTube.”

She relented a bit, acknowledging having expressed her interest in a gas-tax increase if lawmakers would agree to offset it with an income-tax cut.

“So you said, ‘Yes,’” Mr. Scott said.

But Ms. Haley — a more natural political debater — was rolling, and she waved her open palm at Mr. Scott as if she could tamp him back from across the stage they shared with five other Republican candidates.

“On the curtains — do your homework, Tim, because Obama bought those,” Ms. Haley said.

The smiles had vanished, replaced by the corrosiveness of the Republican Party on full display: friends turning on each other to squabble over the cost of window coverings. The exchange underscored the unease inside a party that has shifted over the course of their relationship and now belongs to a man who declined to show up for the debate.

“Did you send them back?” Mr. Scott asked Ms. Haley about the drapes. Mr. Scott’s eyes widened, and he extended his arms at his side as he repeatedly asked if she had tried to return them.

Ms. Haley tilted her forehead toward him, narrowed her eyes and returned the same accusatory question reminiscent of an I-know-you-are-but-what-am-I schoolyard taunt.

“Did you send them back?” Ms. Haley asked. “You’re the one who works in Congress.”

“Oh my gosh — you hung them,” Mr. Scott said, holding his arms in the air to simulate the act of hanging drapes on a curtain rod. “They’re your curtains.”

“They were there before I even showed up,” Ms. Haley said, adding, “You are scrapping.”

“I’m not scrapping,” Mr. Scott said.

The split screen they shared as they pointed at each other on television was a long way from the moment just a decade earlier when they stood side by side in South Carolina. Back then, Ms. Haley introduced him as the best pick to represent the state in the Senate.

“He knows,” she said at the time, “the value of a dollar.”