A huge police union in Las Vegas is beefing with the NFL over one of its new credentialing policies ... and if the matter doesn't get sorted out soon -- it could mean big headaches are on the horizon for the Raiders.

According to Steve Grammas -- the president of the Las Vegas Police Protective Association -- Roger Goodell's org. has implemented a new series of protocols for police members working games this season ... and Grammas says he's got issues with them.

Specifically, Grammas says The Shield wants cops to submit biometric data -- including a picture of themselves -- on an app in order to gain clearance to do their jobs in specific parts of Allegiant Stadium.

"They're going to take your biometric data -- your face," he said in a video sent to Vegas cops this month, "and they're going to use that however they need to. And they're going to extend that to their NFL family partners to use your information should they need to."

Grammas says that could spell big problems for law enforcement members ... claiming the data could end up "in the hands of people who are anti-cop that support a different agenda than what law and order supports."

Grammas recommended the league stick with its old policy -- where it simply gave officers wristbands or proxy cards.

For now, Grammas tells TMZ Sports cops are still going to work Raiders home games -- beginning with their preseason matchup against the Cowboys this weekend -- because submitting their biometric data is not yet a requirement.

However, he said if the policy does get officially enacted by the NFL ... he will advise his cops to no longer work the games -- an issue the Raiders would certainly love to avoid this season.

So far, Grammas says attorneys from both sides are in talks to resolve the problem -- but there's been no resolution yet.