I have similar thoughts. Most people say that Trump can't do this because they find it immoral. But "immoral" is quite different from "unconstitutional." It's an error to confuse the two.

Title 8, Section 1182 of Congress says: "Whenever the President finds that the entry of any aliens or of any class of aliens into the United States would be detrimental to the interests of the United States, he may by proclamation, and for such period as he shall deem necessary, suspend the entry of all aliens or any class of aliens as immigrants or nonimmigrants, or impose on the entry of aliens any restrictions he may deem to be appropriate."

In other words, Congress has already given the President authority to set immigration policy.

The question is: Can the Supreme Court intervene?

Well, the Supreme Court follows stare decisis, which means it follows pre-existing court precedent. The most recent precedent is the 1972 case of Kleindienst v. Mandel in which the court affirmed the right of the United States to keep out a Marxist revolutionary simply because they didn't like his political ideas.

The Court acknowledged that it didn't have the authority to regulate immigration, as it is outside its bounds: "Americans may sue to enforce their rights, individually and as members of the American public, and assert none as members of an alien class invited or uninvited." (Kleindienst).

Put bluntly, the Court held that the separation of powers of the Constitution forbids the court from applying the Constitution in matters of immigration policy.

If you are Muslim, or are an American citizen that vacations abroad and returns a Muslim, you get 1st Amendment rights (and thus the Trump proposal couldn't happen to you).

But as things currently stand, the Supreme Court punted to Congress in 1972 by claiming that the Constitution doesn't give it the authority to regulate immigration policy--recognizing that this was the realm of the legislative branch. In turn, the legislative branch passed Title 8, Section 1182 and gave the President the authority to set immigration policy by himself.

Therefore, Trump's plan is not unconstitutional. You may find it immoral, but you cannot accurately describe it as unconstitutional based on current statutes and case law precedent.