Democrats need 24 seats to win back the House of Representatives and reinstall Nancy Pelosi as Speaker. Although swings of 24 seats are not common, they do happen and there has been two in the last 10 years.
Pelosi first became the Speaker in 2006 when Democrats gained 30 seats in the second George W. Bush mid-term. He was weighed down with the Iraq War and Katrina. But Barack Obama lost 63 seats in his first mid-term election (2010) as the Tea Party gained prominence and influence in the Republican Party fighting Obamacare. John Boehner became the Speaker.
Prior 2010, the largest shift in seats was the Republican revolution in 1994 when Newt Gingrich and the Contract for America won 65 seats and ended 40 years of Democratic control of the House.
Presidential approval is highly related to the president’s party performance in mid-term election years. The general theory has been a surge and decline rotation where a president is elected as his support surges in the polls, but then declines in the off-year election. But especially as presidential approval drops into the forties or below, seats are lost. Republicans in Congress occupy 23 seats that Clinton won in 2016. The Cook Report lists 36 Republican congresspersons in seats where Trump was not particularly popular in 2016. His approval now hovers near 40 percent.
The health care defeat was a demoralizing factor for Republicans, at least for now, just as Democrats appear highly motivated in their opposition to all things Trump.
Dems could take House in 2018