I don't.
Not voting at all at the top of the ticket (I assume that's what you mean by "blank") is the equivalent of being nothing, gone, disappeared, non-existent. Each vote is like an oar in the water of a huge ship. Not voting is withdrawing your oar (so to speak) and not using it at all when you're called upon to help set the course for this ship of state.
I believe that informed folks like us have a DUTY to vote, even when we don't want to. To go blank at the top of the ticket while voting down-ticket would be, to me, a dereliction of duty. Men didn't fight and die for us to suddenly become non-existent at the ballot box.
Enough third party votes -- official ones, not myriad write-ins that could easily invalidate those ballots on the slightest pretense, but marking the ballot at the top of the ticket for any name other than Romney or Obama so that is officially counted in the final tally, would mean VOLUMES to both God and man. VOLUMES.
To God, it would mean that we feared Him more than we feared a mere mortal Obama, and to God it would mean that we loved Him more than we loved our illusions of "pragmatism."
To man, it would mean one more oar's pulling power away from a leftward course, because both Obama AND Romney are heading left.
The term "voting against" is a euphemism for "voting FOR this in order to prevent THAT from winning." Mathematically, there is NO SUCH THING as voting "against," a fact instinctively grasped by those so disheartened that they are willing to just not vote at all at the top of the ticket. When we vote, we can only vote FOR something; the problem with voting FOR Romney in order to vote "against" Obama, is that we'd end up with Romney and those who voted for him to vote "against" Obama would learn, one year into a Romney term, that the only thing that counts is what they voted FOR, because Obama and ABO would be ancient history.
A third party vote has the power to split the end numbers into a plurality that would necessarily weaken the victor. Witness Bill Clinton, whose 43% plurality meant a full 57% of voters essentially thought he stunk. Two years later, Clinton was steamrolled by the Republican Revolution in Congress; the Republican Revolution would probably never have happened if Clinton had won with a majority or if HW had won.
A third party vote forfeits having any voice in influencing whether Obama or Romney wins -- the vote is absolutely neutral on that front, mathematically and materially. HOWEVER, a third party vote at the top of the ticket is the ONLY way you can use that vote to help the conservatives you've voted for down-ticket. Enough third party votes could potentially make it so whichever menace wins, Romney or Obama, takes office with it ON RECORD that nearly two in three Americans rejected him. If the winner happened to be Obama, Obama would be deeply humiliated, scorned, mocked, and vulnerable, especially as the dynamics for Conservatives would be immeasurably strengthened in the Republican party as moderates saw Romney LOSE.
So in fact, a blank or third party vote would mean CONSIDERABLE to both God and man.
I have a responsibility to vote, even at the top of the ticket. I will be voting FOR a plurality, and the only way I can do that is to vote for an official, on-the-ballot third party candidate.
Your Ron Paul, or Virgil Goode, or whoever, “oar” won’t even tickle the “ship of state” as though, as in European systems, the vote were for a party which will get proportional representation.