Colorado's tourists would be able to buy as much marijuana as residents, if a bill moving through the state Legislature passes.

The measure repeals Colorado's unique-in-the-nation tiered purchasing system for marijuana. All adults over 21 here are allowed to possess an ounce of marijuana — but retail pot shops can't sell more than a quarter ounce in one day to people without Colorado IDs.

The purchasing limits were established in 2013 to prevent marijuana diversion out of state. Lawmakers figured that visiting tourists wouldn't blaze through a full ounce before heading home. An ounce of pot is sometimes compared to a keg of beer because it's difficult for most users to finish in a few days.

"We don't want people taking it out of state," said Sen. Cheri Jahn, a Democrat sponsoring a bill that includes the change. "What we found was, that's really not happening."

Colorado pot regulators now say that tourists carrying small amounts of pot home aren't Colorado's main marijuana-diversion problem.

Instead, state regulators and local law enforcement point to underground pot growers as the most likely source of sending pot to states where the drug isn't legal.

Earlier this month, police and federal agents arrested more than 40 people and seized piles of pot plants and elaborate growing equipment during raids of homes and warehouses throughout the Denver area.

Colorado's Marijuana Enforcement Division, which oversees pot shops, has never cited a retailer for selling a tourist more than a quarter-ounce of pot.

But the industry fears that new possession limits on concentrated and edible pot due to take effect this fall would make it harder for retailers to keep track of two limits. That's because the new rules don't consider leafy pot amounts to be the same as, say, pot brownies or hash oil or a skin lotion infused with marijuana's active ingredient.

"We'd have to hire math professors to work in the dispensaries" to keep track of purchasing limits, said Mark Slaugh, head of the Colorado Cannabis Business Alliance.

Another pot seller said the tiered purchasing system makes it harder to complete the detailed inventory requirements Colorado requires.

"It complicates everything," said Mike Lord, director of business operations for LivWell, which has 14 marijuana dispensaries.

The top marijuana adviser to Colorado Gov. John Hickenlooper, Andrew Freedman, said the governor had not yet reviewed the proposed change.

But Freedman pointed out that the changes could complicate enforcement of different limits for tourists and residents.

"Having an enforceable standard is important to us," Freedman said.

Removal of the tiered purchase limits is part of a routine renewal of Colorado's pot regulations. That renewal has passed the state House and is awaiting approval in the state Senate, where an initial committee approved the change Wednesday.