In prior AEDPA cases, the Sixth Circuit used a two-part test for prosecutorial misconduct claims: (1) whether the prosecutor’s remarks are improper, and if so, (2) whether the remarks were flagrant, which the Sixth Circuit determined using four factors.
In Parker v. Matthews, 567 U.S. __ (June 11, 2012) (available here), the Supreme Court described the Sixth Circuit’s two-part prosecutor misconduct test as mere circuit law, and not appropriate for assessing the "highly generalized standard for evaluating claims of prosecutorial misconduct set forth in Darden v. Waignwright [477 U.S. 168 (1986)] ..."
To determine issues of prosecutorial misconduct, the Supreme Court uses the Darden fair trail test: "a prosecutor’s improper comments will be held to violate the Constitution only if they 'so infected the trial with unfairness as to make the resulting conviction a denial of due process.'" The Supreme Court explains, "the Darden standard is a very general one, leaving courts "more leeway . . . in reaching outcomes in case-by-case determinations."