Americans in every state began directly electing their senators in 1914 following the passage of the 17th Amendment. The next 25 presidential elections, from 1916 to 2012, always saw at least some states vote for candidates of different parties for president and Senate. That long streak finally came to an end in 2016, when every single state voted for the same party for both the presidency and the Senate. Democrats won 12 Senate seats, all in states that Hillary Clinton carried, and Republicans won 21 Senate contests, all in states where Donald Trump prevailed. Republicans are also strongly favored to win a December runoff in one more Trump state, Louisiana.
You can see this remarkable set of results in sharp relief in the graph below:
Not only were the outcomes strongly correlated, but so were the actual margins between the parties themselves, as the gray diagonal trendline indicates above. (For stats nerds, the R² value was 0.85, quite a strong correlation.) The horizontal axis shows the Democratic margin against Republicans in the presidential race, while the vertical axis shows the same for the Senate, meaning Clinton and Senate Democrats won those races in the upper-right quadrant, while Trump and Senate Republicans prevailed in those in the lower-left.
Note that this graph excludes California and Louisiana because their all-party primaries meant there was no November matchup between a single Republican and a single Democrat. You can check out the invaluable Dave Leip’s Atlas for full results for both the Senate and presidential races by state. Below we’ll take a look at some of the standout races.
Among the most extreme divergences were Hawaii, where Democratic Sen. Brian Schatz prevailed by 51 points, outrunning Clinton’s margin by 19 percent, and North Dakota, where Republican Sen. John Hoeven romped to a 62-point landslide and exceeded Trump’s margin by 26 percent. Both senators scored the largest margins of victory for any candidate within their respective parties.
The states where winning candidates underperformed their presidential ticket the most for each party were Illinois and Missouri. Democratic Rep. Tammy Duckworth defeated Republican Sen. Mark Kirk by 14 percent, running 3 points behind Clinton’s margin in Illinois. On the other side, Republican Missouri Sen. Roy Blunt survived by just 3 percent against Democrat Jason Kander, lagging far behind Trump’s margin by 16 points, which is a staggering underperformance for an incumbent. Also noteworthy was Kentucky, where Republican Sen. Rand Paul defeated Democrat Jim Gray by 15 points, meaning he ran 15 percent behind Trump’s landslide margin there.
Interestingly, the states with the lowest divergences between presidential and Senate results tended to be competitive states. Taking first prize was Nevada, where Clinton and Democrat Catherine Cortez Masto both prevailed by the exact same 2.4 percent margin. Alabama, Colorado, Nevada, New Hampshire, and Pennsylvania all saw presidential and Senate margins within one percent of one another, and all of those states except Alabama had relatively close races for both contests.
As the 2018 Senate election cycle begins, this pattern of relatively low divergences between the presidential and Senate results should help give us a good idea of which races could be competitive or not. Democrats are defending Senate seats in five states that Barack Obama won in 2012 but that Trump carried in 2016: Florida, Michigan, Ohio, Pennsylvania, Wisconsin. They’re also guarding five more Trump states that Mitt Romney also won four years earlier: Montana, Missouri, Indiana, North Dakota, and West Virginia. Meanwhile, Nevada is the only Republican-held state that voted for both Clinton and Obama.
If 2018 sees a similarly low pattern of split-ticket outcomes like the last several election cycles, Democrats will largely be on the defensive, with very few opportunities to gain the net three seats they’d need to reclaim the majority.