Conservative critics of Trump have proposed that Castle is a better alternative for them than Johnson/Weld. They have a stronger argument than the libertarian critics, though I would suggest that they stay away from Castle as well and seriously consider Johnson.
The key problem with Castle is that he is plainly a candidate of the paranoid conspiratorial far right. His campaign platform focuses on the dark fantasies of the John Birch Society. Many years ago, the John Birch Society claimed that communists held key positions in the U.S. government and they were working on behalf of their foreign masters. There was an element of truth to this claim. There were plenty of communists in the U.S. in the forties and fifties who were taking orders from Stalin's Russia and some of them were in senior government positions.
The John Birch Society went wrong by first claiming that pretty much every center-left politician and bureaucrat in the U.S. was taking orders from Stalin, and then going further and claiming that the same was true of center right politicians like President Eisenhower and Nixon Before long, if you didn't agree with the John Birch Society, you must be part of the international communist conspiracy.
And if that wasn't crazy enough, by the sixties, it was no longer the KGB and the Soviet Union giving the orders, but rather it was a secret cartel of "globalist" bankers giving orders to both the Soviet leadership and pretty much all U.S. politicians conspiring to impose one-world big government.
Key issues in Castle's campaign platform are the struggle against the United Nations and Agenda 21. The problem isn't that the United Nations is particularly valuable or that there is something good about Agenda 21. It is rather that these are big red flags that Castle is not connected with reality. They have no place as key issues facing the U.S. today.
Interestingly, one of the negative aspects of Trump's campaign has been his close ties to conspiracy theorists and really, what seems to be a willingness to take them seriously. That is one key reason why Castle will not get much traction as a practical matter and why it is not desirable that he get any traction.
From a libertarian perspective, the best parts of Castle's campaign platform are his call to abolish the Fed and his support for the U.S. Constitution. The section of his campaign platform that calls for the abolition of the Fed is pretty good. It emphasizes allowing people to use alternative monetary instruments, including Bitcoin. It mentions in passing that dollars would be redeemable in gold. While I think a gold or silver standard is a mistake, many libertarians strongly support a return to the gold standard.
The problem is the audio file linked by Castle to this text that explains why he favors abolishing the Fed. He rapidly heads into bad economic analysis that has a long pedigree in far right conspiracy theory. The Federal Reserve is private. Our money is based on debt. This creates problems because there can never be enough debt-money to pay the interest on the money. Wrong, confused, and paranoid.
Personally, I am more interested in reforming the Fed than abolishing it, though if I had a magic wand, I would abolish it. In the broader libertarian movement, support for reforming the Fed by constraining it with rules has had many important advocates--like Milton Friedman. (I favor replacing the Fed with a rule-constrained monetary authority, so for me the difference between reform and abolition is a matter of a fresh start.)
Castle's devotion to the U.S. Constitution is a positive. I certainly support constitutional restraints on government, and the U.S. Constitution is what we have. But the dominant approach in the modern libertarian movement has been to seek to strengthen the U.S. Constitution, rather than hold it up with an almost religious devotion. And that relates to a fundamental problem with the paranoid right. In their view, the fundamental problem is that the communist (or globalist) traitors have not kept their oath to follow the U.S. Constitution and that is the source of all of our problems. It is part and parcel of their conspiracy theory. Libertarians, on the other hand, have mostly understood that the mainstream of U.S. politics have driven through massive loopholes that were always part of the U.S. Constitution.
Now, in the sixties, the dominant voices in the libertarian movement were all for closing off the loopholes in the U.S. Constitution and providing more express language to protect economic and personal liberties. Later, largely due to the influence of Murray Rothbard, opposition to the U.S. Constitution became more important. Rather than holding up pro-Constitution founders like Madison, other pro-Constitution founders like Hamilton were attacked as advocates of "big-government." Constitution-skeptics among the Founders, the "anti-Federalists" like Patrick Henry, were more in favor. And then there are the anti-Constitution abolitionists of the nineteenth century, chiefly Lysander Spooner that were held up as libertarian heroes. The "radicals" in the libertarian party were strongly influenced by "anarcho-capitalism" and so critics of the general project of constitutionally limited government. These perspectives on the U.S. Constitution remain today as important strains among libertarian scholars and intellectuals.
The tremendous success (by libertarian standards) of Ron Paul's Presidential primary efforts in 2008 and 2012 has greatly strengthened "pro-" U.S. Constitution rhetoric and approach within the libertarian movement. That is because Ron Paul always emphasized his support for the U.S. Constitution. As someone who supported Congressman Paul in those efforts, there is no doubt that there was both a libertarian wing and a conspiracy wing among campaign volunteers.
Is Darrel Castle more libertarian than Gary Johnson? I don't think so. I must admit that Ron Paul's approach has been somewhere between that of Castle and Johnson. Is Ron Paul closer to Castle than to Gary Johnson. I think it depends on what parts of Ron Paul's message you find most important.
Another of Castle's key platform positions is his anti-abortion stance. He shares that view with Ron (and Rand) Paul. Johnson is "pro-choice" on abortion, though he has supported leaving the issue to the states and as a state leader supported some restrictions on late term abortions. In my opinion, Johnson comes closer to the libertarian mainstream on the issue. The Paul's are outliers as are those libertarians whose views on the issue are similar to Clinton.
So, Castle isn't much of a libertarian and while he is a conservative of sorts, I think sensible conservatives should steer clear of the paranoid conspiracy nonsense.
Is the Constitution Party something that should be supported? Not by libertarians, though perhaps some conservatives might find it compatible with their views.
The Constitution Party has seven principles on its website. The first is anti-abortion. In many ways, the Constitution Party has historically been focused on opposition to abortion. There is, of course, a relationship to the U.S. Constitution, since the Supreme Court blocked states from outlawing (or hardly regulating) abortion based upon a Constitutional "right to privacy."
Now, on the whole, libertarians would like to see the Supreme Court protect more liberties, both personal and economic. Still, many libertarians are critical of Rowe vs. Wade. My view is that "grey areas" like abortion are the last place the Supreme Court should be overturning state and local government action.
The second principle is described as liberty and includes personal and religious liberty. It is quite good. Interestingly, it does not speak to so-called "religious liberty" statutes.
The third principle is about the family and is plainly inconsistent with any kind of libertarian approach. Here the Constitution Party plainly states that the the laws should be based upon Christian (and Jewish) scripture. In particular, marriage is ordained by God to be one man and one woman. They claim that state and local governments have the right to restrict offensive sexual behavior. Of course, this is more or less what some conservatives (say Ted Cruz) ran upon, so this might be a positive for some conservatives considering the Constitution Party. (At least the Constitution Party does not support federal legislation to persecute gay people.)
The fourth principle is private property rights. It mostly involves 4th and 5th Amendment protections. In my view it is quite good. Though I am not sure that privacy legislation to prevent private entities from requiring social security numbers is really a priority.
The fifth is interpretation of the U.S. Constitution according to the intentions of the Founders. That is good. The link on the website is broken.
The sixth is states rights. It is a strong statement regarding the 10th amendment. I like it, though I don't like the term "states rights," given its use by advocates of slavery and segregation. I think "federalism" is the better term. And, like many libertarians, I agree with Ayn Rand's statement that states don't have rights, only individuals have rights. Rather than speak of "state's rights," I would say that the U.S. Constitution provides for federalism, which is a good way to protect individual rights.
The seventh principle is American sovereignty. Here there is a very strong support for nonintervention in foreign affairs. It also proposes the withdrawal of the U.S. from just about every international agreement, organization, or treaty. I imagine this goes a bit far for most conservatives, though many libertarians would have no problem with anything in this section. It is too "isolationist" for me. I prefer Rand Paul and Gary Johnson's foreign policy realism in general. And I think some international treaties and agreements are desirable--that is why the Constitution expressly allows the Senate to adopt them. I am not worried much about international organizations. To me, the Constitution Party's focus on them shows paranoid worries about the secret international global conspiracy to imposes one world government.
If you go to the Constitution Party Platform, there are plenty of problems, making it unacceptable for any libertarian. For me, one deal killer is:
"Article I, Section 8 provides that duties, imposts, and excises are legitimate revenue-raising measures on which the United States government may properly rely. We support a tariff based revenue system, as did the Founding Fathers, which was the policy of the United States during most of the nation’s history. In no event will the U.S. tariff on any foreign import be less than the difference between the foreign item’s cost of production and the cost of production of a similar item produced in the United States. The cost of production of a U.S. product shall include, but not be limited to, all compensation, including fringe benefits, paid to American workers, and environmental costs of doing business imposed on business by federal, state, and local governments."
Talk about protectionism! This implies that tariffs must be used so that there is no specialization based upon comparative advantage. The tariffs on bananas sure will be high. Is it based on the cost of operating a banana plantation in south Florida or growing them in hothouses in Alaska?
It goes on: