WASHINGTON (AP) _ The Supreme Court agreed Monday to clarify when a criminal defense lawyer's failure to challenge a sentencing error should be considered constitutionally ineffective legal help.

The court said it will hear a former Chicago union leader's argument that he is entitled to a new sentencing because his lawyer failed to challenge a sentence that was up to 21 months longer than he should have received.

Paul L. Glover, former vice president and general counsel of Chicago Truck Drivers Helpers and Warehouse Workers Union, was convicted of racketeering conspiracy, soliciting and receiving kickbacks, money laundering and tax evasion.

Prosecutors said that between 1986 and 1992, Glover made unauthorized investments with money from union pension, health and welfare funds to get kickbacks for himself and others.

Under federal sentencing guidelines, a federal judge determined his proper sentencing range was 78 to 97 months. The judge sentenced him to 84 months in prison, and appellate courts affirmed his convictions and sentence.

Later, Glover determined that the sentencing range should have been 63 to 78 months, which would have resulted in a sentence 6 to 21 months shorter than what he received. He went to court, arguing that his lawyer's error amounted to ineffective assistance under the Constitution's Sixth Amendment right to counsel.

A federal judge ruled against him in September 1998, saying the potential change in sentence ``must be a significant amount'' to meet the constitutional standard of ineffective assistance. The 7th U.S. Circuit Court of Appeals upheld that ruling in June 1999.

A 1984 Supreme Court ruling said defendants raising ineffective-assistance claims must show their lawyer performed below an objective standard of reasonableness, and that the defendant suffered prejudice because of it.

In the appeal granted review Monday, Glover's lawyer said the 1984 ruling did not require people to show they suffered ``significant'' prejudice because of their lawyer's error. Other appeals courts have ruled for defendants in cases similar to Glover's, his lawyer said.

Government lawyers said Glover could not demonstrate that his lawyer's performance was deficient.

The case is Glover v. U.S., 99-8576.

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On the Net: For the appeals court ruling: http://www.uscourts.gov/links.html and click on 7th Circuit.