Last week, the battle over Amendment 71 on Colorado's fall ballot featured a Greenpeace blimp flying over Denver and Boulder.

And this week, the question of whether to make it tougher to amend the state constitution( full text here) drew football legend John Elway and Hollywood star Mark Ruffalo into the fray.

Elway supports Amendment 71; Ruffalo is opposed.

Ruffalo ("Avengers," "Spotlight") — who joined in an anti-71 social media effort — also has spoken out against the use of hydraulic fracturing by the oil and gas industry.

His entry into the Colorado ballot race is the latest indication that a debate over how to change Colorado’s constitution in some quarters has become an extension of the state’s ongoing fight over the future of its multibillion-dollar oil and gas industry.

Read more at the Denver Business Journal: