Who can declare the acts of congress as unconstitutional

Hey there!
When the founding fathers created the government, they certainly didn't want it to become like the tyranny they opposed. Therefore, they created the three branches of government- where the surprise court could declare congressional acts unconstitutional. Here's an example: if members of a certain party that has ideals that oppose the constitution fill the house or senate and pass unconstitutional laws, the supreme court can call them out.
Hope this helps!

the judicial branch can declare laws passed by congress or executive acts by the president of the united states unconstitutional.

hope this helps, God bless!