After Kamala Harris pulled out of a South Carolina criminal justice forum because its organizer gave Donald Trump an award, the president duly lashed out.

“Badly failing presidential candidate Kamala Harris,” Trump wrote on Saturday morning, “will not go to a very wonderful largely African American event today because yesterday I recieved [sic] a major award, at the same event.”

The California senator trails the frontrunners in the Democratic primary, despite a strong performance in the first debate. She is still in the top five in polling averages and has qualified for the next contest, in Georgia in November.

Her campaign said on Friday she would skip the 20/20 Bipartisan Justice Center event at Benedict College in Columbia, a historically black college, in objection to the group’s decision to give Trump its Bipartisan Justice Award, which she received in 2016 with the Republican South Carolina senator Tim Scott.

Harris also complained that only a handful of Benedict students were given tickets for Trump’s appearance. Most seats were occupied by administration officials and Trump supporters.

Trump received the award for the First Step Act, which has allowed thousands of non-violent offenders to gain early release from federal prison. In a long speech at Benedict on Friday, the president belittled Barack Obama’s record on criminal justice and discussed the impeachment inquiry.

He said on Saturday he won the award because he had been “able to produce [and] sign into law major Criminal Justice Reform legislation, which will greatly help the African American community (and all other communities), and which was unable to get done in past administrations despite a tremendous desire for it.”

The First Step Act was greeted as a bipartisan success but it is not without its critics among campaigners and even those who supported it. In December Kevin Ring, the president of Families against Mandatory Minimums, which backed the bill, told the Guardian it was “a small first step, but it is finally a step in the right direction”.

On Saturday Trump added a familiar claim, writing: “This and best unemployment numbers EVER is more than Kamala will EVER be able to do for African Americans!”

Factcheckers have said Trump’s regular claim about African American unemployment being at its lowest point ever is, at least under modern methodology, accurate – up to a point. He has also regularly claimed strong support among African Americans, a claim not backed up by polling evidence.

Harris hit back on Twitter, writing: “My whole life I’ve fought for justice and for the people – something you’d know nothing about. The only part of criminal justice you can claim credit for is the ‘criminal’ part.”

Harris had been among 10 Democrats expected to attend the forum at Benedict College. Soon after her announcement, the mayor of Columbia, a co-host, said he was organizing an alternative event. Harris’ campaign said she would be there.

On Saturday afternoon, the 20/20 Bipartisan Justice Center was removed as sponsor and attendance was opened to more students. Mayor Steve Benjamin told the Associated Press he was working with college officials to coordinate what was now being called the Collegiate Bipartisan Presidential Forum. The original organisers said they would still have a role.

Shortly before appearing at the forum, New Jersey senator and presidential hopeful Cory Booker said on Twitter: “Donald Trump was given a platform unchecked for close to an hour. The Bipartisan Justice Center allowed him to create some illusion of support from this community when, in fact, he excluded it.”