It's the economy, libertarians.
A weak economy is causing millions to suffer, while nearly every economist who has a good predictive record over the past five years (re: the housing bubble, financial collapse, inflation, interest rates, European GDP growth under austerity, etc), says the US economy right now cannot handle shrinking the public sector or raising taxes on people who use their paychecks to consume (i.e. the non-wealthy).
The reasoning is that there is no private sector demand to fill in for public sector losses, so public sector cutbacks will just cause more weakness in both the public and private sector. After nearly 4 yrs. of European malaise and outright double-dip recessions given public sector cutbacks, this can hardly be denied.
Libertarians, including Gary Johnson, are dogmatic in their desire to cut the public sector no matter the circumstance. But the best prediction, made by those who have the best records, is that significant cutbacks at the federal level in 2013 would immediately push the US back into recession, as it has done in the UK, SPain, Ireland, and Greece.
Here's a few of the bad ideas about public sector spending Gary Johnson has endorsed:
1. Gary Johnson opposed the public infrastructure spending in the American Recovery Act. The US is spending a lower % of its GDP on infrastructure than at anytime since WWII, despite the fact that economists with good predictive records recommend government investment on infrastructure during economic downturns to offset private sector slumps.
2. Gary Johnson opposed federal aid to local and state governments. Even with this aid, state and local governments have fired nearly 700k public sector workers, including hundreds of thousands of teachers. Most economists with good predictive records have pt.ed out that this has been a huge drag on the recovery. It turns out that firing people does not create jobs during recessions.
3. Gary Johnson has proposed cutting military spending by 1/2. While this might or might not be a good idea when the private sector is in good shape, this is a terrible idea currently. It would almost certainly send the US into a deep recession.
In short, it's in almost no one's personal economic interest to vote for an economic illiterate to start setting policy in 2013, and it's certainly not in the interests of the 25 million under and unemployed in this country either.
Pay attention to the economists who have solid prognostication records, have some concern for people who aren't doing well in these poor economic times, and do not vote for Gary Johnson.