House Democrats vying to flip control of the lower chamber in next year’s midterm elections may face a new challenge: the exodus of battleground incumbents seeking higher office. A number of vulnerable House Democrats are eyeing runs for the Senate or other offices in a handful of states around the country, including Michigan and Maine. If they do make the leap, they would vacate tough House seats, eliminate the Democrats’ advantage of incumbency, and force party campaign operatives to recruit new candidates — and spend more money — to keep those districts in the next Congress.