The Volatile Senate: Control of the upper chamber hasn't been this unsteady since the post-Civil War era.

The National Journal reports:

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

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.

A divided country.