The proposed constitutional amendment is backed by prosecutors, sheriffs and police chiefs’ groups.

It would allow past criminal acts — even alleged crimes that didn’t result in convictions — to be used to corroborate victim testimony or demonstrate a defendant’s propensity to commit such crimes when people face sex-related charges involving victims younger than 18.

If approved by Missouri voters in November, Constitutional Amendment 2 could make it more difficult for defendants to persuade juries and judges of their innocence, said Kim Benjamin, a Belton attorney who is the past president of the Missouri Association of Criminal Defense Lawyers.

“You’re now defending your entire life, your entire reputation, rather than this one act,” she said. “It causes a tremendous risk for more people to be wrongly convicted.”

One of Benjamin’s most prominent clients was Burrell Mohler Sr., the patriarch of a western Missouri family who was accused along with his four sons of sexually abusing young relatives over many years. The charges ultimately were dropped in March 2012, after Mohler had spent more than two years in jail while awaiting trial.

The Missouri ballot measure would essentially undo a December 2007 decision by the Missouri Supreme Court, which struck down a state law allowing evidence of past sexual crimes to be used against people facing new sex-related charges involving victims younger than 14.

The court said in that ruling that “evidence of prior criminal acts is never admissible for the purpose of demonstrating the defendant’s propensity to commit the crime with which he is presently charged.”

The Legislature voted last year to refer the proposed constitutional amendment to the 2014 ballot.

Supporters of the ballot proposal have formed a campaign committee with the hopes or running advertisements for it, though the group had barely $1,300 in its account as of the end of June.

Benjamin said defense attorneys have not organized a campaign against the amendment, because she said they lack the money to do so.

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