Ervin and Baker have a joint announcement planned for Wednesday morning in Langley Park.
Their announcement will come on the eve of early voting, which begins Thursday and runs through June 21. Primary election day is June 26.
Ervin could not immediately be reached for comment Tuesday night. A spokeswoman for Baker declined to comment and referred questions to Ervin.
Ervin had a short-lived campaign for the Democratic nomination. She began as the lieutenant governor running mate to Kevin Kamenetz, the Baltimore County executive who was among the leading candidates when he died of sudden cardiac arrest on May 10.