Obviously...

But too obscure apparently to be waived under Gingrich's sausage-like twitter fingers:

I know it's obscure and hidden in the voluminous federal law and Supreme Court Decisions so possibly Newt would have missed it, but there is already fairly well established federal law making it illegal to impose Sharia law on the United States. After an exhaustive search, I found this:

"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances."

Blatantly stolen in its entirety from TPM.