Security guards at MGM Springfield are scheduled to vote on whether to unionize on Tuesday.
The workers could become part of Law Enforcement Officers Security after claims of unfair wage practices have been made.
Union officials describe information sessions being held by the casino about the unionization campaign as union-busting tactics.
In a letter sent to the security staff, MGM Springfield President Mike Mathis is urging employees to vote "no" and says, quote, "Let's tell this union to go away so we can continue to work on our issues".