But under the new congressional map created by redistricting -- the districts where candidates are currently campaigning for seats in the next Congress -- there are just 74 districts that fit that "swing district" bill.
Here's the comparison:
In other words, starting with this election, there are now 15 fewer competitive districts than there were in 2010, and 83 percent of congressional districts now clearly favor either Republicans or Democrats.
Here's what that looks like on the new congressional map:
The result is that the blue districts will, almost without fail, elect liberal Democrats, while the red districts will, almost without fail, elect conservative Republicans. And because these members basically need to please only one side of the aisle to win reelection, their incentive is to toe the party line just about 100 percent of the time.
(A side note: the Fair Vote study also shows the inherent advantage Republicans have in the House, with 195 districts leaning their way, compared with 166 that leans Democrats' way. A big part of this is because Democratic voters are more concentrated in urban areas.)
Redistricting is handled by the state legislatures in the vast majority of states -- which leads lawmakers to draw safe districts for incumbents or, at least, draw districts that their party will be able to win.
There is an emerging movement to put that power in the hands of nonpartisan redistricting commissions. But for now, partisan politics still dominates the drawing of districts.
And the map above is the result.