Sheridan announced his candidacy Sunday for the seat being vacated by retiring Sen. Tim Cullen.
Sheridan was in the Assembly from 2005 through 2010, serving the last two years as speaker.
He lost his Assembly seat after disclosing he was dating a lobbyist for the payday loan industry around the time he softened his stance on regulating that industry.
Sheridan has lobbied for the AFL-CIO since 2012, but quit last month to run for the Senate.
Sheridan is a former General Motors employee and president of United Auto Workers Local 95.
Sheridan faces a primary Aug. 12 against state Rep. Janis Ringhand, of Evansville, and Janesville native Austin Scieszinski.