Before we go too far down the "Mittens Is a Bad Candidate" Rabbit Hole...

112th Congress
Spitballs and Toys left in class

Mittens is not a great candidate.  However, to be honest, it was the best of  dreadful lot.  Before Mittens led the race, Herman Cain (the 9-9-9 Pizza guy) led at one point.  Rick Santorum, who lost his last Senate Race in his home state by 18 pts. was leading.  So, it isn't like Republicans fished the loser from the tank.  The tank was pretty bare.
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But that isn't my point.  My point here is that the Republican Brand blew it after the 2010 election.  The   narrative that the terrible Democrats were messing up the country got a slew of new Republican Congresspeople elected.  But instead of working to fix the country - the double downed on blaming Obama.  They figured to win in 2012, they had to screw the country and blame Obama.
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And here I give the people of the US credit that most people saw through it.
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Rather than work on the Economy, the Republican House of Representatives voted to repeal ObamaCare.  Three times.  To defund it more than those three times.  To remove abortion access to women who's rapes they didn't consider "forcible".  To slash school lunches, and food aid in the middle of a recession.  They voted against ideas they had previously supported - even ones they came up with, if Obama supported them.  The fought against birth control.  Birth Control for heaven's sake!  Come on, that is one thing we all agree stops abortions!  They voted to repeal Don't Ask Don't Tell, even though the Military says it works and hasn't caused the problems they expected in worse case scenarios.
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And they did this time and time again knowing that these wouldn't get voted on in the Senate.  It was a point of pride that the Senate wouldn't even vote on them.
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They have behaved as petulant school children.  And you don't give petulant school children the keys to the family car.
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For many people, and apparently not just Democrats, their last two years isn't indicative of adult behavior.  We didn't elect people to go throw a tantrum.  Well, some in the tea party did, and their representatives promptly gave them what they asked for, theatrics with no results.
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But that isn't the kind of behavior that adults expect of people elected to "Govern".  The screams of "Kenyan", "Muslim" and "Socialist" might have thrilled the base and Rush Limbaugh, it turned off a lot of Independents and moderate votes both Republican and Democratic.
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So the blame isn't all due to Mittens - crazy dog-torturing tax evader that he is.  A big problem is that he has the R after his name and Congress has been trashing the Republican Brand at the national level.