Donald J. Trump’s criminal trial entered a critical and combative phase on Thursday as his lawyer grilled the prosecution’s star witness, Michael D. Cohen, about a medley of misrepresentations, manipulations and outright lies.

Trying to destroy Mr. Cohen’s credibility with the jury, the lawyer, Todd Blanche, portrayed him as an unrepentant criminal and a serial deceiver who took the stand only to exact revenge on Mr. Trump. He argued that Mr. Cohen, Mr. Trump’s loyal lawyer and fixer until a falling-out years ago, had changed his story about matters big and small: whether he had wanted a White House job, whether he had sought a presidential pardon and whether he had actually committed crimes to which he had pleaded guilty.

“There’s no doubt that you know what perjury means, correct?” Mr. Blanche asked Mr. Cohen. The witness, who has admitted lying to Congress and the courts, replied, “I know what perjury means.”

Mr. Cohen’s conduct — and truthfulness — is at the case’s heart. During the 2016 presidential campaign, he made a $130,000 payment to a porn star to suppress her account of a sexual liaison with Mr. Trump, who later reimbursed Mr. Cohen from the White House. Prosecutors accused Mr. Trump, who denies the sex, of falsifying related records so he could cover up the scandal for good.

When the deal leaked anyway, Mr. Trump soon washed his hands of Mr. Cohen, who in turn vowed to turn on his onetime mentor. Mr. Cohen is the only witness offering firsthand knowledge of Mr. Trump’s involvement in the records, which Mr. Trump denies falsifying.