In what circumstances will a prosecutor be able to impeach defense witnesses with their criminal convictions?

In Lopez, the prosecutor impeached defense witnesses with their misdemeanor offenses and their arrests. (People v. Lopez, supra, 129 Cal.App.4th at p. 1520.) In argument, the prosecutor asserted the multiple arrests gave these witnesses "an ax to grind" against the police and made them untrustworthy due to their run-ins with the police; in contrast, the prosecution witnesses had no contact with the police except to call about a disturbance. (Id. at p. 1521.) The reviewing court held it was error to admit the arrest evidence "due to the danger that it would be employed exactly as the prosecutor employed itnot simply to show that the witnesses might be biased against the police, but that each of them has an untrustworthy and criminal character." (Id. at p. 1523.) The court found counsel's failure to object to this inadmissible evidence prejudicial because these defense witnesses were crucial, the prosecutor urged the jury to disbelieve them for improper reasons, and the "unhindered excess undermines our confidence in the outcome." (Id. at p. 1524.)

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