2. What would be the effect on the United States if this decision had not
validated the idea that the Supreme Court has the power to judge
whether acts of Congress are unconstitutional?

This would have made the Congress as the ultimate power in the democratic system.

Explanation:

It is dangerous for the legislature or the executive to not have a system of checks and balances which is maintained by the judiciary in the system of democracy.

This would have been meaningless if  the jury had no power to declare an act from the congress as unconstitutional. As it has this power, the legislature can be kept in check against compromising the constitution and getting away with it if it ever happens.

they believed democracy was worth the fight