We're trying. Seuss is tricky, but we finally have enough info to put something together. Let's give it a shot and see if we can figure out the enigma that is Horton.
Baby Step 1: Horton and Mayzie might just represent two sides of the American people in 1941, as viewed by Teddy Geisel.
Horton, of course, is the interventionist. The faithful interventionists do their duty, even though it might require uncomfortable travel, diet change (poor Horton doesn't seem to eat the whole time he's hatching), life threatening encounters with armed men, and other inconveniences. When the faithful make a commitment, they stick to it.
Mayzie, on the other hand is an isolationist; she's part of the faithless, apathetic bunch. According to Geisel, the faithless don't mind living it up while others are making sacrifices and doing some big-time suffering. They have no sense of duty or honor, and they don't keep their word.
Baby Step 2: In Horton Hatches the Egg, the duty is to the egg/elephant-bird, of course. But what is the egg/elephant-bird in non-Seussville? It can be anything, really. Everybody has their own personal elephant-bird. It's something that's worth making a sacrifice for.
In a political reading, it seems to go something like this:
America, which must not be occupied or damaged by Germany, Japan, and other threats
+ Jews and others suffering at the hands of foreign threats
+ Children everywhere
= Egg/ elephant-bird
According to Seuss, if we're apathetic and don't support the war, we're not being faithful to our homeland, our children, or the suffering people in other parts of the world. But if we are faithful to the egg-elephant bird, we will obtain peace, harmony, and eventually, public affirmation.
Maybe Seuss saw in the war an opportunity to make the world a better place for children—and people in general. Très controversial, no? But Seuss tried hard not to be "preachy" in his kids' books, so if there's any credence to this interpretation, it's not something most little tykes would pick up on. (We'd like to thank Robert Cohen's The Seuss, The Whole Seuss, and Nothing But the Seuss for getting us thinking along these lines.)
Baby Step 3: In this context, we might also see the elephant-bird as a bold statement against racism (which was a favorite practice of the Nazis). Remember, this is 1940. We're talking before the Civil Rights Movement and before Loving v. Virginia (1967), which finally did away with legal restrictions on interracial marriages in the U.S. Horton v. Mayzie would likely have caused a whole new slew of controversies.