When do comments by a prosecutor in her final and rebuttal arguments to the jury in a criminal case that affirm the veracity of the government's chief witness against the defendant, and repeatedly express her opinion on what was 'true' and 'not true,' become unconstitutional vouching for that witness and unconstitutional expressions of personal opinion which deny the defendant's Due Process right to a fair trial?

QUESTION PRESENTED When do comments by a prosecutor in her final and rebuttal arguments to the jury in a criminal case that affirm the veracity of the government’s chief witness against the defendant, and repeatedly express her opinion on what was “true” and “not true,” become unconstitutional vouching for that witness and unconstitutional expressions of personal opinion which deny the defendant’s Due Process right to a fair trial?