SAG-AFTRA, the DGA and the WGA are rallying behind an effort by Jack Kirby's heirs to terminate rights to Spider-Man, The Fantastic Four and the Avengers.

The bid by Jack Kirby's heirs to get the U.S. Supreme Court to hear an appeal that could result in Marvel and other entertainment studios losing full control over Spider-Man, X-Men and Avengers characters continues to pick up steam. The latest is an amicus brief delivered by Hollywood guilds representing actors, directors and writers.

PHOTOS 35 of 2014's Most Anticipated Movies: 'X-Men: Days of Future Past,' 'Mockingjay,' 'Spider-Man 2'

As previously covered, courts have denied efforts by Kirby's estate to terminate copyright grants because the comic legend's work as a freelancer in the middle of the 20th century was deemed to be a "work for hire." As such, Marvel was considered the statutory author of characters in works such as The Incredible Hulk and The Mighty Thor, meaning Kirby never had any termination rights under the 1976 Copyright Act. An appeals court's use of the "instance and expense" test to come to the conclusion that Kirby had merely created "commissioned works" is under attack by, among others, a former head of the USPTO and a former U.S. register of copyrights.

Now that the high court might potentially review working agreements in Hollywood, SAG-AFTRA, the DGA and the WGA are weighing in on what they say is a "critically important case." Lest anyone think that the ability to reclaim rights from studios is something merely for comic book artists, the guilds say the 2nd Circuit's 2013 ruling "jeopardizes the statutory termination rights that many Guild members may possess in works they created."

Similar to the amicus briefs already filed, the guilds argue that works made for hire are the product of traditional employment relationships, and that to extend the interpretation broadly to commissioned works as well would be a consequential power shift in the entertainment industry.

PHOTOS 'X-Men: Days of Future Past' Premiere: Jennifer Lawrence, Hugh Jackman Hit Black Carpet

Most people know that the copyright term has gotten longer and longer over the years, but what is less appreciated is that when Congress extended the term in 1976, it also crafted the termination provision so as to allow authors to reclaim rights over their creations in the latter period of the copyright term. Some say that the decision to do this was a "compromise": Yes, the copyright term became longer, but the benefits of the longer term would potentially go to authors who had given up rights when they were new to the industry and hadn't much bargaining power.

The guilds now say such a compromise is under threat. "By creating an impossible hurdle for creators to overcome, the instance-and-expense test hands purchasers a windfall gift, particularly in light of Congress' extensions of the copyright term in the 1976 Act and the 1998 Copyright Term Extension Act."

Interestingly, the actors, directors and writers use music to showcase the potential stakes of this case.

"For example, a review of Rolling Stone magazine’s top-500 songs of all-time list reveals that nearly 75 percent were created prior to the effective date of the 1976 Act," says the brief. "Of these, nearly 200 were created and released before these changes in the law. While not all of these songs will be subject to these same issues – e.g. some may have been created within a traditional employment relationship – it is inevitable that many share characteristics of the relationship between Kirby and Marvel and thus will be subject to the same uncertainty or inequity as their statutory termination windows approach."

PHOTOS 'Amazing Spider-Man 2' Premiere: Andrew Garfield, Emma Stone Swing Into London

The brief, co-authored by SAG-AFTRA general counsel Duncan Crabtree-Ireland and senior IP counsel Danielle Van Lier and provided below, describes other scenarios before concluding, "Where the scales of justice are tipped so heavily against the creator and the costs of litigation are so great, it all but eviscerates the authorial rights that Congress preserved to creators."

Twitter: @eriqgardner