The N.F.L. is under pressure from falling television ratings, lawsuits over its handling of concussions, and fan opposition to player protests during the national anthem.
Yet investors keep lining up to help start new football leagues. On Tuesday, the longtime N.F.L. executive Bill Polian and the television and movie producer Charlie Ebersol became the latest entrepreneurs to join the fray when they unveiled plans for the Alliance of American Football.
There have been several short-lived football leagues before, including the United Football League, United States Football League and XFL. Like others before them, Polian and Ebersol say they have a formula for success.