It's Pretty Clear
I'll let the Immigration and Nationality Act of 1952 speak for itself:
Section 212
"Whenever the president finds the entry of any aliens or any class of aliens into the united states would be detrimental to the interest of the united states..he may by proclomation and for such period as he shall deem necessary suspend the entry of all aliens"
Even the Washigton Post says what Trump says is "Mostly True"