McCaskill's campaign issued a statement sharing the news that McCaskill's mother passed away Monday at home in St. Louis. The senator canceled campaign events this past week to be with her mother, who was being treated in a St. Louis-area hospital for much of the week.

"I am very sad to announce today the passing of my mother, Betty Anne McCaskill. For some time, mom's health has not been good, and our family takes comfort that she is now at rest," McCaskill said in the statement.

The senator is currently facing a challenge from Rep. Todd Akin (R-Mo.) for her seat. He issued a statement on Monday evening sending good wishes to her family.

"Lulli and I want to express our deepest condolences to Claire McCaskillClaire Conner McCaskillNSA spying program overcomes key Senate hurdle Senate campaign fundraising reports roll in Dems search for winning playbook MORE and her family," he said. "Our thoughts and prayers are with the whole McCaskill family right now."

— Updated at 6:53 p.m.