As Donald Trump prepares to travel to Mississippi for two rallies ahead of Tuesday’s runoff Senate election, the Republican candidate has come under new pressure over reports of involvement with Confederate imagery and history.

Cindy Hyde-Smith, who is white, faces Democrat Mike Espy, who would be the first black US senator from Mississippi since Reconstruction. The runoff is being staged because no candidate reached 50% in the first vote on 6 November. A second Republican candidate running as an independent split the rightwing vote in that poll, making a Democratic gain unlikely.

On the heels of controversy over a videoed remark in which Hyde-Smith joked about attending a “public hanging” – loaded words in a state where lynchings of African Americans were once rife – CNN reported on Saturday that a review of Hyde-Smith’s legislative history found that she “once promoted a measure that praised a Confederate soldier’s effort to ‘defend his homeland’ and pushed a revisionist view of the Civil War”.

Also on Friday, the Jackson Free Press newspaper reported that Hyde-Smith attended a white private school founded in 1970, the year many Mississippi public high schools integrated racially. Yearbook photos from Lawrence Academy, which is now closed, show Hyde was a cheerleader for a team that had a rebel mascot who carried a Confederate battle flag.

The photos surfaced days after other pictures circulated on social media of Hyde-Smith wearing a grey Confederate military-style hat in 2014, when she was state agriculture commissioner and visited Beauvoir, the beachside home in Biloxi that was the last home of the Confederate president, Jefferson Davis.

Hyde-Smith posted the photos of herself at Beauvoir to Facebook, with the caption: “Mississippi history at its best!”

Spokeswoman Melissa Scallan said the Hyde-Smith campaign had no comment about the Beauvoir photos.

Hyde-Smith said the “public hanging” comment was “an exaggerated expression of regard” for a fellow cattle rancher. During a televised debate, she apologized to “anyone that was offended by my comments” but also said the remark was used as a “weapon” against her.

On Saturday, Scallan responded to the report about the candidate’s high school attendance by saying: “In their latest attempt to help Mike Espy, the gotcha liberal media has taken leave of their senses. They have stooped to a new low, attacking her entire family and trying to destroy her personally instead of focusing on the clear differences on the issues between Cindy Hyde-Smith and her far-left opponent.”

Espy, who was US agriculture secretary under Bill Clinton, spoke on Saturday about how he and his twin sister were among 17 black students who integrated the all-white Yazoo City high school in 1969, graduating in 1971. He said he was called “the N-word” many times during integration.

“I guess you could juxtapose my experience with her experience,” Espy said between campaign appearances in Jackson. “If the story is correct, she consciously made a decision to separate, and my parents consciously made a decision to be inclusive.

“So, that’s a Mississippi I want to be a part of, one of diversity, one of inclusion, one of different experiences … I decided to use that very difficult time to learn from and try to reach out to people of all races. So, if you compare me and that experience to Cindy Hyde and her experience, I’d rather have my experience.”

The Senate race is expected to drive a higher-than-usual turnout for a runoff in Mississippi. More than 43,000 absentee ballots have been requested and that number could increase, the state secretary of state’s office said on Saturday. About 69,000 absentee ballots were requested before the 6 November election. There is usually a large decrease in ballots cast between the first election and a runoff.

A video shot on 3 November showed Hyde-Smith talking about “liberal folks” and making it “just a little more difficult” for them to vote. Her campaign said the remark about voting was a joke.

Saturday was the deadline for in-person absentee voting. Several dozen people waited in a line that stretched out of the Hinds county courthouse in downtown Jackson, including a retired home economics teacher, Illinois Cox Littleton, 92, who said she voted for Espy because she considers him “a highly intelligent man”.

Hyde-Smith was appointed as a temporary successor to longtime Republican senator Thad Cochran, who retired in April. The winner of the runoff gets the final two years of a term he started. Hyde-Smith is the first woman to represent Mississippi in Congress. The state last elected a Democrat to the Senate in 1982.

On Sunday morning, Trump tweeted: “I will be in Gulfport and Tupelo, Mississippi, on Monday night doing two Rallies for Senator Hyde-Smith, who has a very important Election on Tuesday. She is an outstanding person who is strong on the Border, Crime, Military, our great Vets, Healthcare & the 2nd A[mendment]. Needed in DC.”