The Bandwagon Fallacy is an argument that appeals to the growing popularity of an idea. This popularity is used as the reason for accepting it as true.
Such an argument is fallacious because popularity may not arise from an actual fact, but may result from peer pressure, political expediency, or even plain mass stupidity. Popularity does not guarantee the truthfulness of an argument.
Creationism is supported by most Americans, so it must be true.