Recently, I attempted to introduce via testimony at jury trial the fact that my client had a pending lawsuit against multiple witnesses who were testifying against him. The case was litigated in the Central District of Illinois federal court. I attempted to elicit such testimony and advised the court at a sidebar of my intent. The government objected stating they believed the information was irrelevant to any issue before the jury. I advised the court that such information should be allowed in to prove bias, prejudice, or motive to slant/lie by the witnesses. The court advised it was not certain if such evidence was allowed in. It asked me to brief them the following morning before beginning the last trial day.
I had admitted such evidence in past cases and was surprised both the government and the judge was unaware of established case law allowing such evidence to be presented. My research began with Alaska v. Davis (S. Ct.) which stated the 6th Amendment Confrontation Clause allows a wide variety of evidence at trial. While specifically not stating pending litigation in another matter is allowed in, it stated the window for evidence bolstering a party's claim should encompass more than just theories the court or opposing party believes is legitimate. I narrowed the search to Illinois and quickly discovered a 2017 Northern District case on point - Leslie v. Roberson (N.D. 2017). The opinion stated that, although in that case it was considered harmless error, the court should have allowed the party to introduce proof of such pending litigation as it could prove malice, prejudice, or bias on behalf of a witness testifying against that person.
State courts have allowed this type of evidence for similar purposes.