Gov. Ron DeSantis of Florida hit back on Friday at one of his leading Republican rivals, Senator Tim Scott of South Carolina, accusing the senator of credulously repeating liberal criticisms over Florida’s educational standards for the teaching of slavery.

A day earlier, Mr. Scott had joined a long list of politicians, educators and historians in criticizing Florida’s new standards for African American history, which include a widely denounced line that middle schoolers should be instructed that “slaves developed skills which, in some instances, could be applied for their personal benefit.”

Speaking to reporters in Iowa, Mr. Scott, the only Black Republican in the Senate, said: “What slavery was really about was separating families, about mutilating humans and even raping their wives. It was just devastating.”

He added, “So I would hope that every person in our country — and certainly running for president — would appreciate that.”

Mr. DeSantis, who is facing rising pressure from Mr. Scott in the unofficial contest to be the leading Republican alternative to former President Donald J. Trump, swiped back on Friday.

Republicans in Washington like the senator, Mr. DeSantis said, “all too often accept false narratives, accept lies that are perpetrated by the left.”

“The way you lead is to fight back against the lies, is to speak the truth,” Mr. DeSantis told reporters in rural Albia, Iowa, during a bus tour of the state. “So I’m here defending my state of Florida against false accusations and against lies, and we’re going to continue to speak the truth.”

The remarks by Mr. DeSantis — who has been in a defensive crouch — plunged him deeper into a fight about slavery and education with two prominent Black Republicans.

On Thursday, he rounded on Representative Byron Donalds of Florida, an ally of Mr. Trump’s, for criticizing the educational standards.

“Are you going to side with Kamala Harris and liberal media outlets,” he asked, nodding to the vice president’s recent critique of Florida’s actions, “or are you going to side with the state of Florida?”

DeSantis allies have defended the governor’s position and the Florida standards, arguing that critics are seizing on a few isolated lines and that mainstream standards have included similar guidance in the past.

Jeremy Redfern, a spokesman for the governor’s office, called attention on Twitter to the official framework for an Advanced Placement course on African American studies that was rejected by Florida, setting off an earlier political controversy over education.

The A.P. framework mentions that “in addition to agricultural work, enslaved people learned specialized trades and worked as painters, carpenters, tailors, musicians and healers.” It adds that “once free,” African Americans “used these skills to provide for themselves and others.”

On Friday evening, both Mr. DeSantis and Mr. Scott, as well as Mr. Trump, Nikki Haley, former Vice President Mike Pence and other Republican candidates, will appear at a Lincoln Day dinner hosted by the Republican Party of Iowa, in a rare convergence of the top tier of the G.O.P. field.

With Mr. Trump leading by more than 30 percentage points in national polls of the race, and holding a comfortable edge in limited surveys of Iowa, the rest of the Republican candidates are jockeying to overtake Mr. DeSantis as his top rival.

And the governor’s position has appeared precarious: His donors and allies have increasingly expressed doubts about his strength as a candidate and his ability to fix his campaign’s problems, among them profligate spending.

Mr. DeSantis also received blowback this week from fellow Republicans for remarks he made about Robert F. Kennedy Jr., a Democrat who supports abortion rights and who has spread conspiracy theories about vaccines. The governor suggested on Wednesday that Mr. Kennedy would be a good option for top posts at public health agencies like the Food and Drug Administration and the Centers for Disease Control and Prevention.

On Friday in Albia, Mr. DeSantis sought to clarify his comments.

“I was asked about appointing him to be like V.P., and I said, he’s liberal so I wouldn’t do that,” he said. “But I do agree with him on Fauci and the lockdowns. The lockdowns were a disaster for this country.”

Mr. DeSantis said that while he would appoint only a “physician or a Ph.D” to a post like director of the C.D.C., he wanted to “work with people across the political spectrum” who agreed with his coronavirus policies.

“So I want Democrats who have been willing to acknowledge the mistakes, to be willing to speak out against that,” he said. “But that’s not the same as appointing to a position.”

Maya King contributed reporting from Ankeny, Iowa.