What is NBC News’ reasoning  about Trump and the GOP not having control of the House of Representatives after the 2018 midterm elections?
Trump’s predecessors, George W. Bush and Barack Obama, did not have low approval ratings this early into their presidencies like Trump does (he sits at 39%). Both Bush and Obama ended up losing control of the House in midterm elections:
There are two ways to look at the new national NBC/WSJ poll  we released Sunday. The first way: President Trump’s firing of FBI Director James Comey didn’t significantly change the president’s overall standing. Trump’s job-approval rating dipped one point from 40% in April to 39% now, which is well within the poll’s margin of error. And his fav/unfav score is 38% positive/52% negative — again basically unchanged from April.
But here’s the second way to view the poll: Trump’s 39% job rating is a screaming alarm bell for the Republican Party when you think about the midterms, which are still more than 500 days away. To put Trump’s 39% into perspective, George W. Bush didn’t reach that level in the NBC/WSJ poll until October 2005, so after the Social Security debacle, after the Iraq war turned south, and after Hurricane Katrina. And the GOP lost the House and Senate the following year. And Barack Obama NEVER reached 39% in our poll — his lowest approval rating was 40% in September 2014, right before Democrats lost the Senate (after losing the House in 2010).