COLUMBIA, South Carolina — Mayor Bill de Blasio got thrown his first curveball in the early days of his quixotic presidential campaign Saturday as a local, prominent Democratic activist urged him not to battle President Trump in the gutter.

“I’m going to tell you the truth, I don’t respect the tit-for-tat and I don’t want our system this time to become a side show,” Beverly Diane Frierson pointedly told Hizzoner, meting out the admonishment as only a retired school teacher could.

“I really don’t because the issues we face are so important and are so crucial that we cannot — in my opinion — allow the foolery of Trump to sidestep the issues.”

De Blasio looked on from the front of a small side room at a public library in the city as Frierson held the room in rapt attention.

“So, please remember, a word of advice, don’t be afraid to deal with him,” she concluded. “But please don’t allow him to get you off message and to sidestep the issues that we face.”

Her rebuke came after de Blasio and President Trump engaged in an extended back-and-forth on Twitter following the mayor’s Thursday announcement he would seek the Democratic nomination to challenge the onetime New York developer in the 2020 presidential election.

During his first two days on the campaign trail, Hizzoner has hammered away on Trump, calling the president “Con Don” for his myriad of business and ethics woes — despite having repeatedly faced pay-for-play allegations of his own back in New York.

The longtime teacher sat in the back of the Saturday meeting of the South Carolina Democratic Party’s Black Caucus, which was first addressed by another 2020 contender — Andrew Yang — and from the moment she rose to address the mayor, it was clear this wasn’t just another question.

“Mine is more of a word of caution if you don’t mind,” she began. “I agree that Trump is a bully and a con artist and I also know that when you all take the debate stage, I know that you cannot be afraid to take it to him.”

Then she proceeded with her blistering critique and demand for more civil politics.

His conduct under question, de Blasio responded by turning to an old crutch, attacking the Democratic establishment.

“If I had to critique our beloved party,” he began, before arguing he wanted to “align himself” with Frierson’s point. “Job 1 is to define ourselves once and for all as on the side of working people, not the elites.

“Once you do that, you’re in the position to mix it up where you have to,” he added, pointing to New York City’s fight with the Trump administration over its sanctuary city policies.

The exchange was the highlight of the hourlong session de Blasio had with longtime party activists — his first in the key state of South Carolina since announcing his candidacy.

Despite the finger wagging, Frierson said afterward that she was impressed with de Blasio and the answer that he provided to her question — an impression that was shared by the smaller-than-usual crowd.

“I’ve heard about 12 or 13 so far,” she said. “[He’s] in the top tier.”

De Blasio had made his entrance into the library just as Yang was finishing a meeting with local activists.

The “Yang Gang” tried to take the party-crashing mayor in stride.

“They’re definitely piggybacking on what we’re doing,” said Jermaine Johnson, a local volunteer organizer who helped put together the event.

It was initially supposed to end at 12:30 p.m. but got stretched by an hour to make room for Hizzoner’s quixotic bid.

Yang is an internet entrepreneur who is running on a platform of providing a universal income.