Aside from bringing back pro football’s formative days, “Leatherheads” might be remembered as the film that permanently drove a wedge between George Clooney and the Writers Guild of America.

Clooney went financial core last fall, after the WGA decided 2-1 in a credit arbitration vote that only Duncan Brantley and Rick Reilly deserved screen credit on the picture that Universal opens today.

Going fi-core means a member is still technically a member of the WGA, but has limited rights within the guild. Fi-core members have to pay dues and are covered by the health and pension plans. Once you elect to go fi-core, the decision is irreversible.

“When your own union doesn’t back what you’ve done, the only honorable thing to do is not participate,” said Clooney, who stressed he made no attempt to exclude Brantley and Reilly.

Clooney says he would have quit the WGA altogether if he could, but that would have prevented him from working on all WGA-covered productions. He says he wanted nothing more to do with the WGA but didn’t want to be hampered in his ability in writing scripts.

As for “Leatherheads,” Clooney took a languishing 17-year old project and got a greenlight after personally giving the script a major overhaul that transformed it into a screwball comedy. He says he felt he’d written all but two of the film’s scenes.

While he agreed that Brantley and Reilly deserved first position credit for hatching the idea and characters, he was incensed enough by the WGA arbitration process to go financial core, which rendered him a dues-paying non-voting member.

The WGA had no comment about Clooney’s decision.

Clooney didn’t appeal the WGA ruling, and kept his action quiet because the WGA was gearing up for a strike at the time. He didn’t want the filing seen as him having split ranks with the union over the labor dispute.

Clooney has been a vocal advocate for urging studios and unions to resolve their differences as soon as possible; he joined Tom Hanks, Sally Field and others at a testy February meeting with SAG leaders in order to urge the guild to start bargaining ASAP.

To Grant Heslov, who partners with Clooney in the production shingle Smoke House, and who was a producer with Clooney on “Leatherheads,” the fi-core move was simply a reaction to a bad WGA decision.

“This script that Duncan and Rick wrote sat languid until after we finished ‘Good Night, and Good Luck’ and George wanted to do something lighter,” Heslov said. “George liked ‘Leatherheads,’ but said it never felt quite right. He took it to Italy with him, and I remember when he called to say he thought he’d solved it. One thing that you clearly see, if you read the original, the subsequent drafts and then his draft, is that he wrote the majority of the film. When I got the call about the decision that he wasn’t getting credit, I was shocked. We both thought Duncan and Rick would get first position credit, which they deserved. But this wasn’t right.”

WGA requires directors who seek writing credit to be responsible for 50% of the script. Heslov said Clooney kept his displeasure quiet because he didn’t want to be viewed as a credit hog since, after all, he is the star, director and a producer of “Leatherheads.” But Clooney confirmed his exit to Daily Variety.

Heslov said this wasn’t about ego, pointing out that when Universal sent a notice that the film would bear the credit “A George Clooney Film,” Clooney nixed it. And while Clooney and Heslov shared an Oscar nomination for original screenplay on “Good Night, and Good Luck,” Clooney and ex-partner Steven Soderbergh removed their names from the producer roster, leaving Heslov the sole nominee when the Clooney-directed pic became a best picture candidate.

“He doesn’t take possessory credit because he believes this is a collaborative business and he’s not a guy who needs credit,” Heslov said. “Financial core was his form of protest, but when he did it, he didn’t want it public. We’re both big union guys. Between us, we belong to 12 unions. I think they made the wrong decision, and he was within his rights to respond by going financial core.”

By going fi-core, writers withhold the portion of dues spent by the WGA on non-contract activities — while still being able to write scripts. Fi-core writers pay dues that are 1.9% less than regular members; they also can’t vote on contracts or in any WGA election.

Under WGA rules, if the director or producer of a film is proposed for final credit, an automatic arbitration is triggered.