Voices & Choices

U.S. Senate Results Show Continued Rise in Predictive Power of Partisanship

U.S. Senate Results Show Continued Rise in Predictive Power of Partisanship
What Election Day showed about the changing face of American democracy

Republicans took the Senate in a wave this November, but the real story of the Senate this year is one of partisan entrenchment: a continued increase in the rise of party affiliation above all other factors as the most important driver of election outcomes.

Democrats defended Senate seats in seven states carried by Mitt Romney in 2012: Alaska, Arkansas, Louisiana, Montana, North Carolina, South Dakota and West Virginia. It appears likely that Republicans will take all of those seats. (Louisiana Democrat Mary Landrieu will face a tough runoff election in December; Republican candidates won the other six.) Out of all 36 Senate races held this year, only three—in Colorado, Iowa, and Maine—were won by a different party than the party of the winning presidential candidate in that state in 2012. And assuming a Republican victory in Louisiana, the new Senate will contain just 16 “crossover” Senators from states that favored the opposite party in 2012’s presidential election. The lesson? States and voters are aligning themselves by partisanship in increasing lockstep.

The fact that this year’s realignment worked to Republicans’ advantage is in large part because this year’s Senate races were concentrated in conservative states to a historic degree. The Monkey Cage’s Patrick Egan notes that the median state holding a Senate election this year voted for Barack Obama by 7.2 percentage points less than his national vote share in 2012, the largest such gap in over six decades. But the real story here is partisanship, regardless of party, and Republicans may eventually find the shoe on the other foot: eleven of the 16 remaining “crossover” Senators are Republicans in states that voted for Barack Obama in 2012, though not all are deep blue (Colorado, Florida, Illinois, Iowa, Maine, Nevada, New Hampshire, Ohio, Pennsylvania, and Wisconsin). Five Democratic crossover Senators remain, in Indiana, Missouri, Montana, North Dakota, and West Virginia.

It’s significant that many of these are perennial presidential battleground states: their partisan composition is close enough to an even split to allow their votes for Senate—like their votes for President—to go either way based on relatively small shifts in the partisan climate, so crossover Senators are an expected phenomenon. In the deep red and blue states, by contrast, the crossover Senators tend to be held tenuously by either vulnerable newcomers or corner-case incumbents. Heidi Heitkamp (D) will face an uphill battle in North Dakota in 2018, having won a razor-thin upset in a strong Democratic year in 2012. Montana’s Jon Tester (D) also scraped out a narrow win in 2012, bolstered by a Libertarian spoiler candidate; he will likely find it difficult to compete in a state that voted for Romney over Obama by 13 points. Meanwhile, blue Maine’s Susan Collins (R) is notably moderate and has held the seat for almost two decades, while red West Virginia’s Joe Manchin (D) is a uniquely conservative Democrat. Though each election cycle has its quirks, these crossover seats are not likely to be sustainable holds in the long term.

The rise of partisanship-as-predictor reflects the increasing ideological divergence between the two parties and the withering of the last vestiges of the parties’ heterogeneity in the twentieth century, such as white Democrats in the South and moderate Republicans in the northeast. This year is a further indication that partisanship is increasingly the dominant driver of federal election outcomes, dwarfing candidates’ attributes, voting records, or campaign tactics and spending. See this analysis from FairVote’s Monopoly Politics report for a fuller look at the dominance of partisanship in national elections—and why it matters.

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