A constitutional amendment isn't necessary to achieve a substantial part of what's necessary for presidential election reform. States choose how to allocate their electors, and could choose to do so proportionally. At least two states already do this. If even just a few key states allocated electors proportionally, the biggest problems with presidential elections would be addressed, specifically, candidates winning the election despite losing the popular vote.

Allocating electors proportionally is probably the easiest path to more sensible elections because states already control this, but more importantly, it's an easy sell to citizens. Convincing citizens of a state to allocate all electors based on the national popular vote despite how its citizens vote is really difficult - no one wants their electoral power to go to a candidate they don't like. The approach has been to get a group of state to agree.

In contrast, convincing citizens to allocate their state's electors proportionally is fairly easy - no one wants their electoral power to go to a candidate they don't like. Support for that doesn't need multiple states to agree. It can proceed individually, and each time it passes, there's an immediate effect. The most important places would be large swing states. It would probably only take Florida, Ohio and Michigan to prevent any realistic chances of an unpopular candidate winning. But you don't need them per se. You could target Georgia, Arizona, Wisconsin, and a few others. Even if just a few states agree, the impact would be very large.