In the International Emergency Economic Powers Act of 1977 the Congress gave the President the power to declare "national emergencies" based on what the "President believes" (section 1703 of the Act). Trump is claiming that Act gives him the power to force all American companies to leave their manufacturing and assembly operations, and their commerce with, China. He is right.

The 1977 law, which was meant to limit presidential power, did no such thing. The law says emergencies can be declared, and almost unlimited presidential power on foreign commerce can be exercised, based on only presidential "beliefs".

Only the weak and frankly vacuous instruction in the 1977 Act to "report to Congress when the President believes" (section 1703), to "consult with Congress", and to "At least once during each succeeding six-month period after transmitting a report" send another report, are the extent of Congressional involvement once a president uses this Act.

The Emergency Powers Act, 1977, pdf.

Trump correctly declares today:

For all of the Fake News Reporters that don’t have a clue as to what the law is relative to Presidential powers, China, etc., try looking at the Emergency Economic Powers Act of 1977. Case closed!

As the New York Times notes:

..the International Emergency Economic Powers Act of 1977, a law originally passed as part of a congressional effort to define and restrain presidential assertions of power not to enable a president to cut off economic ties with a trading partner because of a disagreement over tariffs.

What the law was intended for, and what it says, are two different things.

Trump has already used the Emergency Economic Powers Act to claim imports of Canadian aluminum are a national security threat, and he ordered tariffs on Canadian aluminum,

The Republican Congress did nothing to oppose him.

The Democratic House this year did pass a resolution with the force of law to overturn the Trump Declaration of Emergency on the southern border, and blocking his subsequent use of US Treasury funds for construction of a border wall.

It even passed the Senate with a few Republican votes.  It was vetoed by Trump, his first, making it null and void. It would take a veto override to stop Trump, and enough Republicans were not willing to do that.

Article One section 9 of the US Constitution, says:

No Money shall be drawn from the Treasury, but in Consequence of Appropriations made by Law;

Largely on that Constitutional basis, Congress took the Trump administration to court, arguing the use of military funds appropriated for other purposes, to divert for the Wall, usurped the legitimate power of the House and Congress.

The Supreme Court was not impressed, The Republican judges ruled in a split decision, 5 to 4, that Trump could transfer money appropriated for different reasons, to meet his campaign promise and so-called border emergency. Even though both the House and Senate opposed the action, and opposed the declaration of an emergency.

The Republican justices on the Supreme Court support a very wide scope of power for a Republican president.  They are unlikely to prevent Trump from carrying out his latest threats that US companies must leave China.

The International Emergency Economic Powers Act of 1977, did not limit the President's power, but made it virtually unlimited,  It was missing an important clause, one that said a "presidential belief" that an emergency exists may be overturned by the simple majority vote of either the House or the Senate.

Under current law, and the 1977 Act, as interpreted by the Supreme Court's conservative justices, the court's power does not allow them to second guess presidential powers related to, or the validity of the "beliefs" that a president uses, in declarations of national emergencies.