Hi, thanks for a great question! First, I would like to address how to add amendments to the Constitution, then discuss the two cases you mentioned, and finally answer your question as to why amending the Constitution would be necessary versus passing a law. From Article V of the Constitution, there are two ways to amend the Constitution:
The Congress, whenever two thirds of both Houses shall deem it necessary, shall propose Amendments to this Constitution, or, on the Application of the Legislatures of two thirds of the several States, shall call a Convention for proposing Amendments, which, in either Case, shall be valid to all Intents and Purposes, as Part of this Constitution, when ratified by the Legislatures of three fourths of the several States, or by Conventions in three fourths thereof, as the one or the other Mode of Ratification may be proposed by the Congress; Provided that no Amendment which may be made prior to the Year One thousand eight hundred and eight shall in any Manner affect the first and fourth Clauses in the Ninth Section of the first Article; and that no State, without its Consent, shall be deprived of its equal Suffrage in the Senate.
1)Two thirds of both houses of Congress can propose an amendment and then vote on it. 2)A constitutional convention can be called for by two thirds of the State legislatures.
So, you can see that the proposed amendment becomes part of the constitution when three fourths of the state legislatures ratify the proposed amendment. That would be thirty-eight out of fifty states. Bringing a new amendment into the constitution is pretty hard.
A)Marbury versus Madison.
This landmark case marked the first time the US Supreme Court was granted the right of judicial review. Judicial review just means that the Supreme Court can make null and void any law passed by Congress which is unconstitutional. In 1801, when President Jefferson took office, he refused to allow his Secretary of State, John Madison, to fulfill the commissions given to judges appointed by his defeated opponent, John Adams. As a result, one of the appointed justices, William Marbury petitioned for a writ of mandamus, or legal order, to require Madison to explain why Marbury should be denied his judgeship when the previous President had already appointed him. Chief Justice Marshall ruled that Marbury indeed, had the right. After all, the US Supreme Court should protect the rights of individuals, even if the Court should have to go against a sitting President. Jefferson did not appreciate this at all. However, a silver lining was in store for Jefferson: Justice Marshall ruled the writ of mandamus irrelevant to Marbury's case because it was unconstitutional: Article III states that only certain individuals can directly petition the Supreme Court and these are 'ambassadors, other public ministers and consuls.'
So, although Marbury was eventually denied his commission, the case affirmed the right of the Supreme Court to review any laws passed by Congress or any executive orders issued by a sitting president, an important precedent. If you're interested, read about executive orders and the two executive orders overturned by the US Supreme Court.
B)US Term Limits Inc. Versus Thornton
In the general election of 1992, Arkansas voted in Amendment 73, which prohibited state representatives from serving more than three terms and state senators from serving more than two terms in office. Bobbie Hill and the League of Women Voters sued in protest, and Rep. Thornton was part of this suit. Arkansas attorney general Winston Bryant, along with a non-profit organization named US Term Limits Inc. were supportive of the amendment. The Arkansas Supreme Court ruled for Hill and Thornton; this led US Term Limits Inc to appeal to the US Supreme Court. By a 5-4 decision, the US Supreme Court agreed with Hill and the League of Women Voters: no where does it say in Article 1, Sections 2 and 3(the part of the Constitution which pertains to both houses of Congress) that Congress or states can impose term limits on anyone running for office. The dissenting minority opinion of the Supreme Court, however, warned against rendering states powerless to hold their representatives and senators accountable to the people.
So, how do these two landmark cases illustrate for us two reasons why amending the US Constitution would be necessary versus passing a law:
1)Marbury versus Madison brings up a thorny issue: should a sitting president be able to infringe upon our rights by the simple act of issuing an executive order? Our constitution was originally crafted to limit a monarch/king's abuses of power. Since Article III of the Constitution states that only certain individuals can directly petition the US Supreme Court, and Article VI, Clause 2(Supremacy Clause) affirms the Constitution as the supreme law of the land, Marbury would have needed an amendment to the constitution in order for the Supreme Court to hear his writ of mandamus challenge. Congress passing a law would not satisfy the constitutional test.
2)Term Limits Inc versus Thornton brings up a very important issue. What happens when Congress abuses its power and becomes draconian or worse, corrupt? How would the people reclaim their prerogative of freedom from tyranny in such a situation? Like the Marbury case, this case also demonstrates that passing a law to limit congressional terms would not be enough:the law itself, even if passed, would be unconstitutional because of Article 1, Sections 2 and 3. Hence, an amendment to the constitution would be needed.
Hope this helps. Thanks for the question! There are two links below which you might be interested in.