The 83-year-old Democrat is in his 15th term on Capitol Hill.
Former Cook County Commissioner and Davis staffer Richard Boykin has already filed paperwork to run for the seat.
Davis is a representative for the 7th District of Illinois.
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A source with direct knowledge of the endorsement said Davis will endorse State Rep. La Shawn Ford at a news conference Thursday.
The political landscape in Illinois will undergo a dramatic shift in this next election cycle, which will bring about a significant "changing of the guard" in the Democratic Party, particularly in Congress.
Sen. Dick Durbin's decision in April not to seek reelection was the first political domino to fall.
Congresswoman Jan Schakowsky's retirement announcement followed.