Future Forward Party (FFP) leader Thanathorn Juangroongruangkit apologised to former prime minister Thaksin Shinawatra for his remarks concerning Thaksin's "conflicts of interest" as he testified before the Constitutional Court in a media-shareholding case on Friday.
The court has scheduled a ruling in the case for Nov 20 after hearing witnesses on Friday.
The Election Commission (EC) brought the case against Mr Thanathorn for allegedly holding shares in a media company in violation of the constitution.
Mr Thanathorn told the court that: "I am determined to pursue politics without the conflicts of interest that got Thaksin Shinawatra [in trouble].
"I don't want to seek benefits or surround myself with an entourage of people like Thaksin. I want to change this society. If it stays the same like this, it cannot move forward," he said.
After the hearing, Mr Thanathorn tweeted an apology to Thaksin. "When I came back and listened to what I said about Mr Thaksin during the hearing, I admit that it was inappropriate of me to mention a third party like that.
"I intended to say that when I decided to leave my business and enter politics, society will be keeping its eyes on me, so I am trying to set a higher standard than the law requires. But I got a bit carried away in the courtroom and I communicated it wrongly."
His remark about Thaksin upset the Pheu Thai Party. Lt Sunisa Divakorndamrong, the party's deputy spokeswoman, said Mr Thanathorn should have focused on his defence rather than speaking about Thaksin.
Mr Thanathorn was the first of 10 defence witnesses scheduled to testify.
The EC alleges he held shares in V-Luck Media Co when he registered as an MP candidate in early February, a violation of Section 98 (3) of the charter, which prohibits owners and shareholders of media and/or publishing firms applying to become MPs.
The leader of the country's third-largest party in term of MPs was suspended by the Constitutional Court from his MP duties on May 23, pending a court ruling on whether on the matter.
Mr Thanathorn repeatedly told judges he could not remember specific details about a matter that is threatening to jeopardise his political future.
The judges asked him to verify if he had transferred his 675,000 shares in V-Luck Media to his mother, Somporn, on Jan 8 as he claimed.
He was also asked whether he had made an appointment in Bangkok to transfer the shares before, or after, he began planning his election campaign in the northeastern province of Buri Ram.
Mr Thanathorn told the judges he could not remember. When the court repeated the question, he replied: "Can you remember if you set one meeting before or after another meeting on a day?"
He said that on Jan 8 he left Buri Ram at 11am and arrived at his residence at Lakeside Villa in Bangkok at 4pm to sign the share transfer contract with his mother.
Judges asked how he cashed the 6-million-baht cheque, Mr Thanathorn said he did not remember because he had his wife handle all his financial transactions.
Asked why the cheque was not cashed until May, Mr Thanathorn said he did not know and had never asked because his family had no financial problems. The late action was his wife's decision, he said.
Mr Thanathorn told the court that despite his shareholding he had never managed V-Luck Media. He had received the shares from his mother only because she wanted his wife to have a job after she had resigned from a bank to raise his children.
Asked why he failed to send the financial reports of V-Luck Media to the court, Mr Thanathorn replied emotionally that he had forgotten because "I did not think whether to send them or not would be significant to the case."
Asked repeatedly to elaborate on the circumstances of the closure of V-Luck Media, Mr Thanakorn said emotionally, "How many times do I have to repeat that I do not remember?"