Democrats have little margin for error to keep control, even as they simultaneously will be working against a redistricting cycle that is likely to favor Republican officeholders. The Democratic departures are likely to make it easier for sometimes-partisan mapmakers to draw maps that favor Republican pickups. They also mean that Democrats will not fully take advantage of incumbency, with its fundraising and name recognition benefits. In 2018, the last midterm shake-up, 91 percent of incumbents won reelection, according to the Center for Responsive Politics. This time, Democrats will be the ones fighting historical head winds that tend to punish the president’s party in midterm elections. Since 1910, the party in the White House has gained House seats in a midterm only twice: in 1934, after the election of President Franklin Delano Roosevelt, and in 2002, when President George W. Bush was leading a response to the terrorist attacks of Sept. 11, 2001. President Bill Clinton lost 54 seats in his first midterm. Barack Obama lost 64. Donald Trump gave up about 40. Amid these grim odds, retirements have long been viewed by party strategists as a key early metric of just how challenging an election cycle will be.