The US had to declare war without Congress, because Congress basically agreed to create a unified Korea under a democratically elected government... an election that the communists won and caused the US military to panic.
As the US had just finished WW2 and Cold War propaganda had not really taken effect yet, the population was vehemently against any other war, especially as the Soviets and Chinese were still seen as good friends and allies. A problem the government... circumvented.