According to a new piece by Amy Walter of The Cook Political Report, Donald Trump may have a negative impact on vulnerable Republicans in the 2018 midterm elections:

The more the GOP gets bogged down in process instead of progress, the more likely it is that their voters become disillusioned and that independent voters abandon them. Combine these ingredients with an energized Democratic base and you have all the ingredients for a disastrous midterm election in 2018 for the GOP.

Walter identifies two key figures that may help determine upcoming danger for the Republican House of Representatives:

...if you look back at the last four midterm elections where the party in the White House lost control of one or both houses of Congress, you see that they share the following traits in common: the president has approval ratings among his own partisans under 85 percent and approval ratings among independents in the 30’s or low 40s.

This is a case in which correlation is assumed to be causation. While it could be true that these two factors are essential in determining a political party's fate in midterm elections, it's also possible that Republicans could defy such trends in 2018 the way Trump defied expectations in 2016.

Elections aren't easily-calculated affairs. Wins and losses aren't simply tied to two prime factors, but to other, smaller determinants as well. For example, Democratic turnout during midterm elections is historically lower than that of Republicans. On the other hand, the extraordinary animus progressives hold against Trump may push Democratic voters to the polls in 2018.

Looking once again to the numbers, Pew Research cites the findings of Brown University's Brian Knight, which show that, statistically speaking, the Republican Party has an uphill battle in 2018:

...since 1842 the President’s party has lost seats in 40 of 43 midterms — the exceptions being 1934, 1998 and 2002.

Knight adds that the "'presidential penalty,' or general preference among midterm voters for expressing dissatisfaction with the president’s performance or ensuring that his party doesn’t control all the levers of government" plays a significant factor in midterm elections.

The final factor to consider is Trump himself. The president's erratic behavior, and his targeting of Republican Party members who refuse to toe the line may also contribute to a general Republican loss in 2018. Independents and political naïfs may see the struggle between conservatism and Trumpsim as tiresome, and look elsewhere for leadership.

2018 could follow in the footsteps of the norm-shattering election year that just occurred, though it's more likely going to be the election that settles the United States back into well-worn trends.