His attorney said Acevedo has been diagnosed with dementia, and is not competent to testify.
The attorney did not provide medical records.
The judge said Wednesday he weighed Acevedo's credibility and competency and said he is required to testify.
The judge also told Acevedo that if he provides truthful testimony and answers to questions that may incriminate him, it will not be held against him.
The government has applied immunity to him.
Mike Madigan trial live updates: Former state Rep. Acevedo to testify in ex-speaker trial
He's expected to testify Monday.
Prosecutors say Acevedo, a Madigan ally, received tens of thousands of dollars from AT&T in 2017, as the utility company sought to curry favor from Madigan to get important legislation passed.
Acevedo's testimony is expected to be the prosecution's final phase in the case at the Dirksen Federal Building.
The judge denied a defense team request to dismiss a count against Madigan Tuesday.
Madigan and co-defendant Mike McClain face bribery and racketeering charges.
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