What was the primary legal argument made by Henderson in his defense against the Minnesota statute regarding slow-moving vehicle signs?

The primary legal argument made by Henderson in his defense against the Minnesota statute regarding slow-moving vehicle signs was centered on an infringement of his First Amendment right to freedom of religion. In this context, Henderson likely argued that the requirement to display certain signs could interfere with his religious beliefs or practices. The First Amendment protects not only the right to free speech, but also the free exercise of religion, allowing individuals to express their beliefs without government interference.

The focus on religious expression illustrates how legal challenges can arise from state regulations that seem to conflict with an individual's deeply held beliefs. Courts often evaluate these cases by considering the extent to which a law affects the practice of religion and whether it imposes a substantial burden on those with sincere religious objections.

The other potential arguments—such as violation of free speech or issues regarding interstate commerce—though they could be relevant in other contexts, do not align as closely with the core of Henderson's defense based on religious belief.