With respect to the necessity of a constitutional amendment, I agree with you on presidential elections but respectfully disagree as to congressional elections.
For presidential elections, the proposal with the most traction is the National Popular Vote Interstate Compact, which requires compacting states to give their electoral votes to the presidential ticket with a plurality of votes nationwide but only takes effect after states collectively possessing a majority of all electoral votes join the compact. Proponents argue that it is constitutional (with many believing it can be done without congressional consent), while opponents say that it is unconstitutional and in any case would require congressional consent. See pages 21-30 of this Congressional Research Service report for a summary of the legal issues. Regardless of which side has the better argument, it’s unlikely that an interstate compact would be used to adopt instant runoff voting or approval voting for presidential elections because i) absent a law from Congress, it would be up to non-compacting states whether to switch from plurality voting in their own state (which could mean voters in some states would be limited to choosing one ticket) and ii) it is questionable whether Congress has the power to require non-compacting states to switch (though see pages 16-17 of this article arguing that it does).
As for congressional elections, it’s worth noting that the U.S. Constitution does not require plurality voting and does not even require single member districts. Indeed, ranked choice voting was used in Maine for congressional elections in 2018, and a federal judge rejected the argument that it is unconstitutional due to being contrary to historical practice. And while single member districts have been used uniformly for nearly two centuries, it was not the only method in use at the founding and courts tend to give special weight to founding era practice (see e.g. Evenwel v. Abbott for an example related to elections), which makes me think that FairVote’s single transferable vote proposal is on solid constitutional footing.
With respect to the necessity of a constitutional amendment, I agree with you on presidential elections but respectfully disagree as to congressional elections.
For presidential elections, the proposal with the most traction is the National Popular Vote Interstate Compact, which requires compacting states to give their electoral votes to the presidential ticket with a plurality of votes nationwide but only takes effect after states collectively possessing a majority of all electoral votes join the compact. Proponents argue that it is constitutional (with many believing it can be done without congressional consent), while opponents say that it is unconstitutional and in any case would require congressional consent. See pages 21-30 of this Congressional Research Service report for a summary of the legal issues. Regardless of which side has the better argument, it’s unlikely that an interstate compact would be used to adopt instant runoff voting or approval voting for presidential elections because i) absent a law from Congress, it would be up to non-compacting states whether to switch from plurality voting in their own state (which could mean voters in some states would be limited to choosing one ticket) and ii) it is questionable whether Congress has the power to require non-compacting states to switch (though see pages 16-17 of this article arguing that it does).
As for congressional elections, it’s worth noting that the U.S. Constitution does not require plurality voting and does not even require single member districts. Indeed, ranked choice voting was used in Maine for congressional elections in 2018, and a federal judge rejected the argument that it is unconstitutional due to being contrary to historical practice. And while single member districts have been used uniformly for nearly two centuries, it was not the only method in use at the founding and courts tend to give special weight to founding era practice (see e.g. Evenwel v. Abbott for an example related to elections), which makes me think that FairVote’s single transferable vote proposal is on solid constitutional footing.