Are Republicans going to lose the House in 2018? And if they do, will it be President Trump’s fault?It’s impossible to know the answer to the first question with any certainty at this point. While major indicators show that Democrats are in good shape for 2018 (polling, incumbent retirements, and election results since 2016 all point in that direction), there’s time for national conditions to improve (or further deteriorate) for the GOP.But it is possible to get handle on the second question.Last summer, I found an interesting pattern in the historical polls. During the presidencies of George W. Bush, Bill Clinton, and Barack Obama, the incumbent party often had trouble outpacing presidential approval in generic ballot polls during midterm elections. That’s why the 1994 Democrats, the 2006 Republicans, and the 2010 and 2014 Democrats all had trouble in mid-term House electionsâthe president was unpopular and many voters didn’t want to support his party (note that 1998 and 2002 are excluded because the aftermath of Monica Lewinsky and the September 11 attacks caused unusual trends in presidential approval).