As an institution, the Senate is built to be stable. One-third of the body is up for a vote every two years, meaning that a majority of senators can't be replaced in any given cycle. Which means, in turn, that elections like the one that's looming in November -- an election in which not one but two senate seats that have been held by one party for more than a century might change party control -- are very rare.

Until early in the 20th century, senators were elected by state legislators, not voters. Which meant that the early history of the body was rather spotty. We compiled the following graphic showing the history of each senate seat in each state. For any year in which a senator was in place for more than half of the year we assigned a Democratic, Republican, or other value to seat, based on the party that controlled the seat for the majority of that time.

What's interesting to look at are the patterns: the emergence of the two-party system, with sporadic election of independents and third-parties. And the gap during the Civil War, with Confederate states rejoining the union only after a few years of absence.

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What prompted our look at the Senate's history was the turmoil in Kansas, which puts incumbent Sen. Pat Roberts(R) at risk -- and raised the possibility at least of breaking the nearly eight decade long stranglehold Republicans have had on the Sunflower State's two Senate seats.

But notice the state that's in second. West Virginia is almost certain to see its nearly six decades of Democratic control evaporate in the next Congress with Rep. Shelley Moore Capito (R) a strong favorite to win the seat of retiring Sen. Jay Rockefeller (D).

Neither of those streaks is close to the record, however. Thanks to robust Democratic dominance in the South until after the Civil Rights era, that party held both Senate seats in a number of Southern states for over a century.

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Roberts' is actually the Kansas seat that's been in Republican hands longer -- almost a century. But two other current senators now sit in seats that have been in their party's hands for even longer. Sen. Mary Landrieu's (D-La.) seat has been held by Democrats for over 130 years; Sen. John Walsh's (D-Mont.) for over a century. Walsh's seat will almost certainly flip this year; Landrieu will be lucky to hold hers.

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All three of those seats were likely due for a change. On average, the Democratic and Republican parties have held their senate seats for just over two decades -- a decent length of time, but not close to a century.

An interesting footnote here. There are only two seats in the Senate that have never been held by one of the two parties. The first is Sen. Brian Schatz' (D-Hawaii), which was held by Daniel Inouye for most of the state's existence until his death in 2012. The other is Sen. Bernie Sander's (I-Vt.) seat -- which has never been held by a Democrat. (Sanders caucuses with Democrats in the Senate but identifies himself as a socialist.)

Since the founding of the Republic, Democrats have held seats in the senate for slightly more cumulative years than Republicans.

When you compare the percentage of years that a state's senate seats have been held by Democrats versus Republicans, Roberts' Kansas is the most Republican in history. The most Democratic? Arkansas -- where Sen. Mark Pryor (D) could very well be ousted in November.

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