This article is from the archive of our partner National Journal

It's not exactly the seven-year itch. But a similar dynamic is looming over November's struggle for Senate control.

Minority Leader Mitch McConnell (Jim Watson/AFP/Getty Images)If Democrats lose their majority this year, it will extend a striking pattern: Since 1980, neither party has controlled the Senate for more than eight consecutive years. That persistent volatility marks a distinct change from most of the 20th century. Given the underlying trends in voting behavior, it's likely the Senate will continue to experience fragile and fleeting majorities. And that points toward both more partisan conflict and mounting pressure to rewrite Senate rules—like the filibuster—in ways that strengthen the majority.

Instability is a constant in the modern Senate. After the 1980 election, Republicans controlled the chamber for six years (until 1986); Democrats held it for eight (through 1994); and Republicans regained control for six, until the 2000 election divided the chamber exactly 50-50. The next two years saw the parties trading the gavel (as a GOP defector provided Democrats a temporary majority) before Republicans regained control in the 2002 election. But they held that advantage only until 2006, when Democrats won the majority Republicans are now threatening.

This turbulence contrasts with the 26 years of uninterrupted Democratic Senate control from 1955 through 1980. Earlier in the 20th century, Republicans and Democrats each ran off 14-year streaks of unbroken control. From 1895 through 1912, Republicans posted an 18-year reign.

Control of the Senate hasn't been this unsteady since the decades after the Civil War. It's no coincidence that was another period, like ours, of intense economic and social upheaval as America was reshaped by reinforcing waves of industrialization, immigration, and urbanization. And yet even then, the Senate wasn't unstable for as long as it has been since 1980.

The Senate is now flipping so frequently partly because neither party can amass much of a cushion even when it gains control. From 1959 through 1980, the Democrats held a Senate majority larger than 10 seats in all but one of the 11 sessions; since then, the majority party has accumulated an advantage that large only three times in 17 sessions. These thin margins have left the governing party unable "to sustain their majority ... when things go badly," notes Brookings Institution senior fellow Thomas Mann.

For decades, each side's partisan advantage in the Senate has proven evanescent.

But, ironically, the margins are so small mostly because the territory each party commands is now so large. Each side dominates the Senate seats in the states that also usually support it for president. Some senators always surmount these trends to win, in effect, behind enemy lines. But overall, Democrats control 43 of the 52 Senate seats in the 26 states that twice backed President Obama. Republicans hold 34 of the 44 seats in the 22 states that twice rejected him. (Two states reversed from supporting to opposing Obama from 2008 to 2012, and they split their four Senate seats.) Extending the lens, Republicans control almost three-fourths of the Senate seats in the 25 states that have preferred their candidates in at least four of the past six presidential races; Democrats hold more than four-fifths of the seats in the 23 states that have supported their presidential nominees that often.

November seems certain to widen this divergence. Democrats face some danger in the two-time Obama states (with New Hampshire, Iowa, and possibly Colorado or Michigan at risk). But their vulnerability is concentrated in the states that twice opposed him: After November, Republicans might control 40 of the 44 seats in these reliably red states.

With each party consolidating Senate seats in its presidential strongholds, the prognosis is for narrow Senate majorities tipped by a few swing states and the handful of senators who win on the other side's natural terrain. Looking forward, the Senate's "natural division ... is very close to 50-50," says Emory University political scientist Alan Abramowitz.

Narrow majorities inherently encourage partisan conflict: When control is always within reach, the minority party loses incentive to help mint legislative accomplishments that fortify the brittle majority. "We used to call it a permanent campaign, but [now] it's an unending war that's been created by these narrow majorities," Mann says.

These electoral patterns promote confrontation in another respect. Because more senators are representing states that also usually support their side's presidential nominee, they face heightened pressure to reflexively back a president from their party—and oppose one from the other. As this dynamic constricts the opportunity for compromise, it seems inevitable that the majority party will further retrench the filibuster—which now induces stalemate far more than it does genuine negotiation.

Most analysts favor the GOP to recapture the Senate this year, largely because Democrats must defend so many seats in states that rejected Obama. But even if Democrats lose control, the blue-tilting 2016 Senate map could allow them to quickly regain it. For decades, each side's partisan advantage in the Senate has proven evanescent. The real trend is the volatility itself—and the spiraling confrontation that it feeds.

This article is from the archive of our partner National Journal.

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