COLUMBUS, Ohio—Democratic lawmakers on Thursday are mulling over a Republican congressional redistricting proposal that would make it more likely for Republicans to pick up two U.S. House seats starting with next year’s midterm elections, according to three sources familiar with the situation.

Ohio House and Senate Democrats have yet to officially sign off on the deal, which would give Republicans an advantage in 12 of the state’s 15 congressional districts, several sources said.

It remains to be seen whether House and Senate leaders, as well as rank-and-file lawmakers from both parties and redistricting commission members, will get behind the map currently being discussed.

Under the plan, two congressional districts currently held by Democratic U.S. Reps. Marcy Kaptur of Toledo and Gre

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