For good or bad, I believe that's what many Americans think/vote when it comes to giving one party the Presidency. After eight years (two terms) they tend to not like (and blame - fairly or not) the party that's been in control of the White House, and switch to the other party.
There are a lot of factors at work in this, from the media wanting fresh "kill", a sense of weariness with the party in power, a desire by people for change, the fact that the other party is on "offense" for capturing the Presidency, and complacency on the party that's been in the White House for eight years.
Then there is the big one: the question "Are you better off now than you were eight years ago?".
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I realize it might seem early to be thinking about 2016, but you can be sure the GOP is thinking about it and already planning. The elections this fall will be their first act in the plan to re-take the White House. And if they can capture the majority in the Senate, that will ensure that President Obama is a lame-duck for the last two years (it normally happens anyway).
I think Democrats, progressives and liberals are going to have to start planning now for ways to counteract the "eight is enough" narrative. I'm not worried about the die-hards among us who will turn out and work and vote. I'm worried about the average person who doesn't follow politics closely and doesn't give things a lot of thought. They just show up and vote every four years for the President that they think they want.
There are a lot of things that could combine for a "perfect storm" of electing a Republican President in 2016: Re-districting, voter and voting suppression, a highly motivated, angry/hate-filled and well-financed GOP, international troubles, Faux News, a declining but still influential Tea Party, a still too high unemployment rate. All that plus the "eight is enough" factor could deliver it to the party opposite.
Of course there are a lot of things that could keep the GOP out of the White House in 2016 - primarily putting up an extremist and/or "looney-tunes" candidate. I just don't think we should count on the other side screwing up in some way, or putting up a bad candidate.
I hope that in 2016 "that eight is NOT enough", but we have a lot of work to do to make sure that happens.