The Rugby League Supporters Association (RLSA) are asking all 12 Super League clubs and the 19 Northern Ford Premiership outfits to make loyalty pledges.
The move follows a rallying call by Hull chief executive Shane Richardson in the wake of a claim by Leicester chief executive Peter Wheeler that several Super League clubs had begun talks about changing codes.
Richardson said: "People should come out now and say where they stand.
"You can't sit on the fence on this one. You're on the side of rugby league or of rugby union.
"Unless people are willing to commit to rugby league in the form of a 10-year loyalty contract, they should tell their fans now."
A spokesman for the RLSA said: "To allay fears over the future conduct of our member clubs, the Rugby League Supporters Association Online will be sending a fax to every member club of the Super League and Northern Ford Premiership over the next two days.
"We request that all supporters should do their best to ensure their club's chief executive signs and returns this fax."