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.

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.